Memory system and data transmission method

ABSTRACT

A memory system of a high-speed operation can be realized by reducing an influence of reflection signals etc. caused by branching and impedance mismatching in various wirings between a memory controller and a memory module, and an influence due to transmission delays of data, command/address, and clocks in the memory module. To this end, a memory system comprises a memory controller and a memory module mounted with DRAMs. A buffer is mounted on the memory module. The buffer and the memory controller are connected to each other via data wiring, command/address wiring, and clock wiring. The DRAMs and the buffer on the memory module are connected to each other via internal data wiring, internal command/address wiring, and internal cock wiring. The data wiring, the command/address wiring, and the clock wiring may be connected to buffers of other memory modules in cascade. Between the DRAMs and the buffer on the memory module, high-speed data transmission is implemented using data phase signals synchronous with clocks.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/593,405 filed Nov. 6, 2006, which is a divisional of U.S. Pat. No.7,155,627 granted Dec. 26, 2006, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a memory system having a configurationthat enables high-speed operation, and further relates to a datatransmission system that is used in the memory system.

2. Description of the Related Art

Conventionally, in the memory systems of this type, interfaces have beenstudies that enable operations at high speed and with low signalamplitude. As a standard for such interfaces, SSTL (Stub SeriesTerminated Transceiver Logic) has been proposed. Further, with respectto the memory systems having DRAMs as memory devices, there have beenproposed such memory systems employing a DDR (Double Data Rate) systemwherein a data transmission speed can be twice by inputting/outputtingdata synchronously with both edges of rise (leading edge) and fall(trailing edge) of clocks, thereby to operate the DRAMs at high speed.

Conventionally, as a memory system employing the foregoing SSTL and DDR,there has been proposed such a memory system wherein a plurality ofmemory modules are mounted on a mother board, and these memory modulesare controlled by a memory controller called a chipset. In this case, aplurality of DRAMs are mounted on each memory module.

As a memory system of this type, JP-A-2001-256772 (hereinafter referredto as “Reference 1”) discloses a memory system wherein a plurality ofmemory modules each mounted with a plurality of DRAMs are mounted on amother board. The disclosed memory module comprises a plurality of DRAMsarranged on a rectangular memory module board in parallel in alongitudinal direction thereof, and a command/address buffer and a PLLchip for distributing clocks to the DRAMs, which are disposed betweenthe DRAMs. Each DRAM on the memory module board is connected to moduledata wiring extending in a short-side direction of the module board,while the command/address buffer and the PLL chip are connected tomodule command/address wiring and module clock wiring extending in theshort-side direction of the module board. Further, for distributingcommands/addresses and clocks to the DRAMs from the command/addressbuffer and the PLL chip, module command/address distributing wiring andmodule clock distributing wiring are drawn out in the longitudinaldirection of the module board.

In this configuration, data signals are directly given to the DRAMs oneach memory module from a memory controller provided on the motherboard, while command/address signals and clock signals are given to theDRAMs on each memory module from the memory controller via thecommand/address buffer and the PLL chip, respectively. In the memorysystem using the foregoing memory modules, when the single memory moduleis taken into consideration, it is hardly necessary to form branchwiring on the memory module relative to signal wiring on the motherboard. Therefore, there is a merit that it is possible to reducewaveform distortion or disturbance due to undesirable signal reflectioncaused by branch wiring. Further, there is also a merit that access timecan be shortened.

JP-A-H10-293635 (hereinafter referred to as “Reference 2”) discloses amemory system wherein a memory controller and a plurality of memorymodules are mounted on a mother board. The disclosed memory systemensures a setup time and a hold time of each memory module to enablehigh-speed signal transfer by matching propagation times of clocksignals and data signals outputted from the memory controller. Further,Reference 2 also describes a method of stably feeding clocks.Specifically, clocks that have twice inputted clocks in frequency areproduced, and signals and outputs of SDRAMs are controlled synchronouslywith the produced clocks in a memory module or memory LSI. In thisconnection, Reference 2, FIG. 28, shows a configuration wherein clockshaving a frequency of 2Φ are produced at the memory controller, and theclocks are divided to half in frequency so as to be clocks having afrequency of Φ, then transmitted to the memory module.

Further, Reference 2, FIG. 34, shows a configuration wherein the clockfrequency given from the memory controller is made twice and fed tomemories in the memory module. Accordingly, Reference 2 discloses atechnique wherein clocks of a predetermined frequency aretransmitted/received between the memory controller and the memorymodule, and the frequency of the clocks is increased twice in thememories such as SDRAMs or the memory controller. In other words,Reference 2 describes that the frequency lower than the clock frequencywithin the memory is transmitted/received between the memory module andthe memory controller.

In Reference 1, the module data wiring extending in the short-sidedirection of the module board, and the module command/addressdistributing wiring and the module clock distributing wiring drawn outonto the DRAMs from the command/address buffer and the PLL chip havedifferent lengths from each other. Therefore, data arrives at each DRAMat timing that differs from arrival timing of command/address and clocksignals, and thus, it is difficult to adjust the timing therebetween.

On the other hand, in Reference 2, inasmuch as the clocks having thefrequency lower than the clock frequency within the memory module aretransmitted/received between the memory controller and the memorymodule, a data transfer time is prolonged. Further, in the configurationof Reference 2, since the transfer speed of data can not exceed theoperation speed of the memory, there arises a limitation about thespeedup and the number of memory modules that can be mounted. Inaddition, Reference 1 and 2 teaches nothing about a technique oftransmitting data at high speed between the memory controller and thememory module.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a memorysystem that can easily adjust timing between data signals, andcommand/address and clock signals in each memory module.

It is another object of the present invention to provide a memory systemthat can reduce reflection signals caused by branching and impedancemismatching and, as a result, that can operate at high speed.

It is still another object of the present invention to provide a datatransfer method that can transfer data at high speed between twocircuits provided in a module.

A specific object of the present invention is to provide a data transfermethod that can transfer data at high speed between a buffer and DRAMsin a memory module.

According to the present invention, there is obtained a memory systemwherein a buffer having a predetermined function is mounted on a memorymodule, and point-to-point connection is provided between a memorycontroller and a memory module and between memory modules. According tothis configuration, signal quality at high frequencies can be improved.Further, signal wirings between the buffer and DRAMs on the memorymodule can be connected using wiring layout that includes onlyelectrically ignorable branching and does not have electricallyinfluential branching, which results in improvement of the signalquality.

Further, according to the present invention, a higher speed memorysystem can be realized by using a data transmission/reception systememploying bidirectional data phase signals on each memory module.

Here, explanation will be given about a buffer according to the presentinvention. The buffer or buffers are provided on a memory module. Datawiring between a memory controller and a memory module or between memorymodules is connected to a buffer on the memory module in a groupedfashion. In a memory system provided with a plurality of memory modules,buffers on the adjacent memory modules are connected to each other viadata lines in a point-to-point fashion. In this case, a data signal istransferred on the data lines at a speed that is n times relative to adata frequency of a DRAM. Further, when compressed into packets andmultiplexed, the number of data lines is reduced to about 1/n (notnecessarily 1/n because there is actually an indivisible case or thelike).

On the other hand, command/address wiring is connected to the memorycontroller and between the buffers of the memory modules per group ofdata wiring and, like the data wiring, is point-to-point connectedbetween the memory controller and the memory module and between thememory modules. A command/address signal is transferred at a speed thatis m times a command/address signal frequency of the DRAM and, whencompressed into packets, the number of signal lines is reduced to about1/m (also not necessarily 1/m because there is actually an indivisiblecase or the like).

The buffer provided on each memory module has a function of receiving adata signal or a command/address signal from the memory controller orthe memory module of the prior stage, encoding packets of the data orcommand/address signal to provide the number of signals corresponding tothe DRAMs on the subject memory module, and transmitting them to theDRAMs at a 1/n or 1/m times frequency. Further, the buffer also has afunction of transferring or transmitting a command/address signal to thecascade-connected memory module of the next stage, and a function ofbidirectionally transmitting/receiving a data signal relative to thenext-stage memory module. The respective signals on the memory modulesare connected with wiring layout having only such branching that can beelectrically ignored. Identification of a packet transmissiondestination of the data or command/address signal is carried out using amodule ID signal.

Characteristic aspects or modes of the present invention will beenumerated hereinbelow.

According to a first mode of the present invention, there is obtained amemory system having a module mounted with a plurality of memorycircuits, and a controller for controlling the plurality of memorycircuits, characterized in that the module is mounted with at least onebuffer connected to the controller via data wiring for datatransmission, and the buffer and the plurality of memory circuits areconnected to each other via internal data wiring in the module.

According to a second mode of the present invention, there is obtained amemory system according to the first mode, wherein the module is mountedwith a plurality of buffers, and the plurality of buffers are connectedto the controller via the data wiring.

According to a third mode of the present invention, there is obtained amemory system according to the first or second mode, wherein the bufferis further connected to the controller via command/address wiring andclock wiring.

According to a fourth mode of the present invention, there is obtained amemory system according to the third mode, wherein the buffer isconnected to the memory circuits via internal command/address wiring andinternal clock wiring corresponding to the command/address wiring andthe clock wiring, respectively.

According to a fifth mode of the present invention, there is obtained amemory system according to the fourth mode, wherein the internalcommand/address wiring and the internal clock wiring are commonly usedfor the memory circuits.

According to a sixth mode of the present invention, there is obtained amemory system according to any one of the first to fifth modes, whereineach of the memory circuits is a DRAM, and data is transmitted/receivedbidirectionally in the data wiring between the controller and thebuffer.

According to a seventh mode of the present invention, there is obtaineda memory system having a plurality of modules each mounted with aplurality of memory circuits, and a controller for controlling thememory circuits of the plurality of modules, characterized in that eachof the modules is provided with at least one buffer, and the buffer ofeach module is connected to the buffer of another module and/or thecontroller via data wiring for data transmission.

According to an eighth mode of the present invention, there is obtaineda memory system according to the seventh mode, wherein the buffer ofeach module is connected to the buffer of another module and/or thecontroller via command/address wiring and clock wiring.

According to a ninth mode of the present invention, there is obtained amemory system according to the seventh or eighth mode, wherein the datawiring forms a daisy chain by connecting the buffers of the plurality ofmodules and the controller in cascade.

According to a tenth mode of the present invention, there is obtained amemory system according to the seventh mode, wherein each of the buffersof the plurality of modules is directly connected to the controller viathe data wiring.

According to an eleventh mode of the present invention, there isobtained a memory system according to the tenth mode, wherein each ofthe buffers of the plurality of modules is further directly connected tothe controller via command/address wiring and clock wiring.

According to a twelfth mode of the present invention, there is obtaineda memory system according to the eleventh mode, further comprisingbuffers provided on other modules and each connected to one of thebuffers in cascade via data wiring, command/address wiring, and clockwiring.

According to a thirteenth mode of the present invention, there isobtained a memory system according to any one of the eighth to twelfthmodes, wherein the memory circuits of each module are grouped into aplurality of ranks, and the memory circuits, belonging to the same rank,of the plurality of modules are simultaneously accessible.

According to a fourteenth mode of the present invention, there isobtained a memory system according to the twelfth or thirteenth mode,wherein a data transmission speed on the data wiring is higher than adata transmission speed on internal data wiring between the buffer andeach of the memory circuits on each module.

According to a fifteenth mode of the present invention, there isobtained a memory system according to the fourteenth mode, whereintransmission speeds on the command/address wiring and the clock wiringare higher than transmission speeds on internal command/address wiringand internal clock wiring, corresponding to the command/address wiringand the clock wiring, between the buffer and the memory circuits on eachmodule.

According to a sixteenth mode of the present invention, there isobtained a memory system according to the fourteenth mode, wherein datafor the buffers of the plurality of modules are transmitted in the datawiring in the form of a packet, and the buffers separate the data in theform of the packet.

According to a seventeenth mode of the present invention, there isobtained a memory system according to the fifteenth mode, whereincommands/addresses and clocks for the buffers of the plurality ofmodules are transmitted in the command/address wiring and the clockwiring in the form of packets, and each of the buffers has a function ofseparating the commands/addresses and dividing the clocks in frequency.

According to an eighteenth mode of the present invention, there isobtained a memory system having a module mounted with a buffer and amemory circuit connected to the buffer, and a memory controllerconnected to the buffer on the module, characterized in that atransmission speed between the memory controller and the buffer ishigher than a transmission speed between the buffer on the module andthe memory circuit connected to the buffer.

According to a nineteenth mode of the present invention, there isobtained a memory system according to the eighteenth mode, wherein aplurality of modules each having the buffer and the memory circuit areprovided, and the buffers of the respective modules are connected inturn in cascade relative to the memory controller via data wiring,command/address wiring, and clock wiring, and wherein the memory circuitand the buffer are connected to each other on each module via internaldata wiring, internal command/address wiring, and internal clock wiring,and transmission speeds on the data wiring, the command/address wiring,and the clock wiring are higher than transmission speeds on the internaldata wiring, the internal command/address wiring, and the internal clockwiring.

According to a twentieth mode of the present invention, there isobtained a memory system according to the nineteenth mode, wherein thememory circuit of each module is a DRAM, data phase signals aretransmitted bidirectionally between the buffer and the DRAM on eachmodule at timing that avoids collision therebetween, and each of theDRAM and the buffer produces internal clocks based on the received dataphase signal and performs reception/transmission of data according tothe internal clocks.

According to a twenty-first mode of the present invention, there isobtained a data transmission method for transmitting/receiving databidirectionally between a first and a second device, the first devicereceiving data according to first internal clocks, and the second devicereceiving data according to second internal clocks, characterized inthat a first and a second data phase signal are continuously transmittedbidirectionally on the same wiring between the first and second devicesat timing that avoid collision therebetween, the first device refers totiming of the first data phase signal to thereby transmit data to thesecond device, while the second device refers to timing of the seconddata phase signal to thereby transit data to the first device.

According to a twenty-second mode of the present invention, there isobtained a data transmission method according to the twenty-first mode,wherein the second device produces the second internal clocks accordingto the received first data phase signal and receives the data from thefirst device according to the second internal clocks, while the firstdevice produces the first internal clocks according to the receivedsecond data phase signal, produces the second data phase signalaccording to the first internal clocks, and receives the data from thesecond device according to the first internal clocks.

According to a twenty-third mode of the present invention, there isobtained a data transmission method according to the twenty-first ortwenty-second mode, wherein the first device suppresses, of the firstand second data phase signals transmitted bidirectionally, the firstdata phase signal outputted from the first device, while the seconddevice suppresses, of the first and second data phase signalstransmitted bidirectionally, the second data phase signal outputted fromthe second device.

According to a twenty-fourth mode of the present invention, there isobtained a data transmission method according to any one of thetwenty-first to twenty-third modes, wherein the first and second devicesare a buffer and a DRAM, respectively, and the DRAM is given externalclocks and produces the second internal clocks based on the externalclocks and the received first data phase signal.

According to a twenty-fifth mode of the present invention, there isobtained a data transmission method according to any one of thetwenty-first to twenty-third modes, wherein the first and second devicesproduce the first and second internal clocks from the second and firstdata phase signals using DLLs.

According to a twenty-sixth mode of the present invention, there isobtained a data transmission system for transmitting/receiving databetween a first and a second device, characterized in that atransmission side of the first and second devices has means fortransmitting, upon transmission of the data, a data phase signalrepresenting a predetermined phase of the data continuously irrespectiveof transmission of the data, and a reception side of the first andsecond devices has means for reproducing internal clocks of thereception side based on the data phase signal and receiving the dataaccording to the reproduced internal clocks.

According to a twenty-seventh mode of the present invention, there isobtained a data transmission system for transmitting/receiving databidirectionally between a first and a second device, characterized inthat each of the first and second devices has transmission means fortransmitting, upon transmission of the data, a data phase signalrepresenting a predetermined phase of the data continuously irrespectiveof transmission of the data, and transmitting the data based on the dataphase signal, and reception means for reproducing data receptioninternal clocks based on the data phase signal and receiving the dataaccording to the reproduced internal clocks.

According to a twenty-eighth mode of the present invention, there isobtained a data transmission system according to the twenty-seventhmode, wherein the first and second devices are a buffer and a DRAM,respectively, transmission means of the buffer has means for outputtinga write data phase signal to the DRAM as the data phase signal,reception means of the buffer has means for receiving a read data phasesignal from the DRAM as the data phase signal, reception means of theDRAM has means for reproducing the data reception internal clocks fromthe write data phase signal, and means for receiving the data accordingto the reproduced internal clocks, and transmission means of the DRAMhas means for outputting a read data phase signal as the data phasesignal at timing relying on the received write data phase signal.

According to a twenty-ninth mode of the present invention, there isobtained a data transmission system according to the twenty-eighth mode,wherein the write data phase signal and the read data phase signal arebidirectionally transmitted onto the same signal line at mutuallydifferent timings.

According to a thirtieth mode of the present invention, there isobtained a data transmission system according to the twenty-eighth mode,wherein the write data phase signal and the read data phase signal arebidirectionally transmitted onto mutually different signal lines atmutually different timings.

According to a thirty-first mode of the present invention, there isobtained a data transmission system according to any one of thetwenty-eighth to thirtieth modes, wherein the read data phase signalreception means of the buffer has means for reproducing data receptionbuffer internal clocks based on buffer internal clocks and the read dataphase signal, and the read data phase signal output means of the DRAMhas means for reproducing DRAM internal clocks for outputting the readdata phase signal, based on external clocks and the write data phasesignal.

When speeding up the foregoing memory systems, it is preferable toemploy the following configurations taking into account a skew on eachmemory module.

Specifically, according to a mode of the present invention, there isobtained a memory module having a plurality of memory circuits and abuffer, wherein a command/address signal is transmitted from the bufferto the plurality of memory circuits, and data signals following thecommand/address signal are transmitted/received between the buffer andthe plurality of memory circuits, characterized in that at least one ofthe plurality of memory circuits and the buffer has skew absorbing meansfor absorbing timing skews that are generated between thecommand/address signal and the data signals depending on mountingpositions of the memory circuits. When each of the memory circuits is aDRAM, it is preferable that the command/address signal is outputtedsynchronously with buffer clocks outputted from the buffer to the memorycircuits.

When employing such a configuration, it is preferable that the skewabsorbing means are provided in the plurality of memory circuits and thebuffer, respectively, and that the data signals are transmitted/receivedbetween the plurality of DRAMs and the buffer synchronously with dataphase signals representing phases of the data signals.

Here, it is preferable that each of the DRAMs is given a command/addresssignal from the buffer synchronously with the buffer clocks and furthergiven a write data phase signal (WDPS) from the buffer as the data phasesignal, and the skew absorbing means of the DRAM has means for producinga plurality of phase clocks for receiving the command/address signalaccording to the buffer clocks, means for producing data reception DRAMinternal phase clocks from the WDPS, and means for domain-crossing thecommand/address signal received synchronously with the phase clocks, tothe data reception DRAM internal phase clocks.

On the other hand, the DRAM outputs a read data phase signal (RDPS) tothe buffer as the data phase signal, and the skew absorbing means of thebuffer has means for producing data reception buffer internal phaseclocks from the RDPS received from the DRAM, means for producing bufferinternal phase clocks based on the WDPS, and means for causing a readdata signal inputted synchronously with the RDPS, to match with thebuffer internal phase clocks.

According to another mode of the present invention, there is obtained amemory module, wherein the DRAM is given a write data phase signal(WDPS) from the buffer as the data phase signal, and inputted with adata signal synchronously with the WDPS, and the skew absorbing means ofthe DRAM has means for producing data reception DRAM internal phaseclocks from the WDPS, means for producing a plurality of phase clocksfrom the buffer clocks, and means for domain-crossing a data signalreceived synchronously with the data reception DRAM internal phaseclocks, to the plurality of phase clocks.

Here, it is preferable that the DRAM outputs a read data phase signal(RDPS) based on buffer clocks, and the skew absorbing means of thebuffer has means for producing data reception buffer internal phaseclocks based on the RDPS, means for producing buffer internal phaseclocks based on global clocks, and means for causing a data signal readfrom the DRAM and received according to the data reception bufferinternal phase clocks, to match with the buffer internal phase clocks,thereby to perform domain crossing.

According to still another mode of the present invention, there isobtained a memory module having a plurality of memory circuits and abuffer, wherein a command/address signal is transmitted from the bufferto the plurality of memory circuits, and data signals following thecommand/address signal are transmitted/received between the buffer andthe plurality of memory circuits, characterized in that the data signalsare transmitted/received between the plurality of memory circuits andthe buffer synchronously with data phase signals transmitted onto thesame signal line alternately from the memory circuits and the buffer,and the buffer has means for outputting a control signal for defining atransmission time of the data phase signal in each of the memorycircuits and the buffer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram for explaining a memory system according to afirst preferred embodiment of the present invention;

FIG. 2 is a schematic stereoscopic wiring diagram for explaining anactual configuration of the memory system shown in FIG. 1;

FIG. 3 is a sectional view for explaining the wiring of the memorysystem shown in FIGS. 1 and 2 more specifically;

FIG. 4 is a block diagram showing a memory system according to a secondpreferred embodiment of the present invention;

FIG. 5 is a schematic stereoscopic wiring diagram showing the memorysystem shown in FIG. 4;

FIG. 6 is a block diagram showing a memory system according to a thirdpreferred embodiment of the present invention;

FIG. 7 is a block diagram showing a first modification of the memorysystem according to the third preferred embodiment of the presentinvention;

FIG. 8 is a block diagram showing a second modification of the memorysystem according to the third preferred embodiment of the presentinvention;

FIG. 9 is a block diagram showing a third modification of the memorysystem according to the third preferred embodiment of the presentinvention;

FIG. 10 is a block diagram showing a fourth modification of the memorysystem according to the third preferred embodiment of the presentinvention;

FIG. 11 is a block diagram for explaining a transmission system betweena memory controller and a buffer in the first to third preferredembodiments of the present invention;

FIG. 12 is a time chart for explaining an operation of the transmissionsystem shown in FIG. 11;

FIG. 13 is a time chart for explaining an operation, upon writing, ofthe transmission system shown in FIG. 11;

FIG. 14 is a time chart for explaining an operation, upon reading, ofthe transmission system shown in FIG. 11;

FIG. 15 is a time chart for explaining an operation, associated with acommand/address signal, of the transmission system shown in FIG. 11;

FIG. 16 is a block diagram for explaining a transmission system betweena buffer and a DRAM, which is used in the memory systems according tothe first to third preferred embodiments of the present invention;

FIG. 17A is a time chart for explaining a write operation in thetransmission system of FIG. 16;

FIG. 17B is a time chart for explaining a read operation in thetransmission system of FIG. 16;

FIG. 18 is a block diagram for explaining a transmission system of thepresent invention that enables speedup of the transmission systemexplained with reference to FIG. 17;

FIG. 19 is a circuit diagram showing configurations of driver portionsof a buffer and a DRAM employing the transmission system of FIG. 18;

FIG. 20 is a circuit diagram showing other configurations of driverportions of a buffer and DRAMs employing the transmission system of FIG.18;

FIG. 21A is a time chart for explaining a write operation when thetransmission system of FIG. 20 is employed;

FIG. 21B is a time chart for explaining a read operation when thetransmission system of FIG. 20 is employed;

FIG. 22 is a time chart for schematically explaining a timingrelationship among signals in the transmission system of FIG. 18;

FIG. 23 is a block diagram for explaining a configuration of a DRAM thatcan realize the transmission system shown in FIG. 18;

FIG. 24 is a block diagram for explaining a configuration of a bufferthat can realize the transmission system shown in FIG. 18;

FIG. 25 is a timing chart for explaining a timing relationship upon thestart of operation in the DRAM shown in FIG. 23;

FIG. 26 is a timing chart for explaining a timing relationship during anormal operation in the DRAM shown in FIG. 23;

FIG. 27 is a time chart for explaining a timing relationship uponreading in the buffer shown in FIG. 24;

FIG. 28 is a block diagram showing an example of a DRAM that can realizea transmission system according to the present invention;

FIG. 29 is a block diagram of a buffer that can performtransmission/reception of a signal relative to the DRAM shown in FIG.28;

FIG. 30 is a time chart for explaining an operation of the DRAM shown inFIG. 28;

FIG. 31 is a block diagram for explaining a modification of atransmission system between a buffer and a DRAM;

FIG. 32 is a timing chart for explaining an operation, upon reading, ofthe DRAM shown in FIG. 31;

FIG. 33 is a timing chart for explaining an operation, upon writing, ofthe DRAM shown in FIG. 31;

FIG. 34 is a block diagram for concretely explaining a configuration ofthe DRAM shown in FIG. 31;

FIG. 35 is a block diagram for concretely explaining a configuration ofthe buffer shown in FIG. 31;

FIG. 36 is a timing chart for explaining a timing relationship in theDRAM and the buffer shown in FIGS. 34 and 35;

FIG. 37 is a timing chart for explaining an operation of the DRAM shownin FIG. 34;

FIG. 38 is a timing chart for explaining an operation of the buffershown in FIG. 35;

FIG. 39 is a block diagram showing another example of a DRAM applicableto the transmission system shown in FIG. 31;

FIG. 40 is a block diagram showing an example of a buffer that cancooperatively work with the DRAM shown in FIG. 39;

FIG. 41 is a block diagram for explaining a memory module according toan example of the present invention;

FIG. 42 is a block diagram for explaining a DRAM that is used in amemory module according to a first example of the present invention;

FIG. 43 is a block diagram for concretely explaining a domain crossingcircuit in the DRAM shown in FIG. 42;

FIG. 44 is a block diagram for explaining a buffer forming the memorymodule according to the first example cooperatively with the DRAM shownin FIG. 43;

FIG. 45 is a block diagram showing a domain crossing circuit in thebuffer shown in FIG. 44;

FIG. 46 is a timing chart for explaining write operations of the bufferand the near-end DRAM that are used in the memory system shown in FIGS.42 and 44;

FIG. 47 is a timing chart for explaining write operations of the bufferand the far-end DRAM that are used in the memory system shown in FIGS.42 and 44;

FIG. 48 is a time chart for explaining a read operation between thefar-end DRAM and the buffer;

FIG. 49 is a timing chart for explaining an operation of the buffer uponreading;

FIG. 50 is a timing chart for explaining an operation of the buffer whenreading out read data from the near-end and far-end DRAMs;

FIG. 51 is a block diagram showing a DRAM that is used in a memorysystem according to a second example of the present invention;

FIG. 52 is a block diagram showing a concrete configuration of a domaincrossing circuit used in the DRAM shown in FIG. 51;

FIG. 53 is a block diagram showing a buffer forming the second exampleof the present invention cooperatively with the DRAM shown in FIG. 51;

FIG. 54 is a block diagram showing a concrete configuration of a domaincrossing circuit used in the buffer shown in FIG. 53;

FIG. 55 is a timing chart for explaining a write operation between thebuffer and the near-end DRAM in the second example;

FIG. 56 is a timing chart for explaining a write operation between thebuffer and the far-end DRAM in the second example;

FIG. 57 is a timing chart for explaining a read operation between thebuffer and the far-end DRAM in the second example;

FIG. 58 is a timing chart for explaining an operation of the buffer whenprocessing read data signals from the near-end and far-end DRAMs;

FIG. 59 is a block diagram for explaining a memory system according to athird example of the present invention;

FIG. 60 is a block diagram showing a configuration of a DRAM that isused in the example shown in FIG. 59;

FIG. 61 is a block diagram showing a configuration of a buffer used inthe third example;

FIG. 62 is a timing chart for explaining an operation in the thirdexample;

FIG. 63 is a timing chart for explaining a case wherein an operationduring initialization of the DRAM and an operation during a normaloperation thereof differ from each other in the third example;

FIG. 64 is a block diagram for explaining a memory system according to afourth example of the present invention;

FIG. 65 is a time chart for explaining a write operation in the memorysystem shown in FIG. 64;

FIG. 66 is a time chart for explaining a read operation in the memorysystem shown in FIG. 64;

FIG. 67 is a block diagram for explaining a memory system according to afifth example of the present invention;

FIG. 68 is a time chart for explaining a write operation of a first DQchannel portion in the memory system shown in FIG. 67;

FIG. 69 is a time chart for explaining a read operation of the first DQchannel portion;

FIG. 70 is a time chart for explaining a write operation of a second DQchannel portion in the memory system shown in FIG. 67; and

FIG. 71 is a time chart for explaining a read operation of the second DQchannel portion.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, there are respectively shown a wiringdiagram and a stereoscopic diagram of a memory system according to afirst preferred embodiment of the present invention. Further, FIG. 3 isa partial sectional view of the memory system for explaining the wiringat a portion of FIGS. 1 and 2 in detail.

As seen from the figures, the memory system according to the firstpreferred embodiment of the present invention comprises a memorycontroller 101 and a clock generator 102 (FIG. 1) that are mounted on amother board 100. Further, on the mother board 100, a plurality ofmemory modules 103 (four memory modules 103 a, 103 b, 103 c, and 103 din FIGS. 2 and 3) are mounted via module connectors 104 (FIG. 3).

Each memory module 103 (subscript omitted) is provided on a module boardthereof with a buffer 105 and, as shown in FIGS. 1 and 2, a plurality ofDRAMs 110. In the shown example, each memory module 103 has one buffer105, and the memory controller 101 and the buffers 105 are connectedtogether via data wiring (DQ) 111, command/address wiring (Cmd/Add) 112,and clock wiring (CLK/CLKB) 113. As clear from this, the data wiring 111shown in FIGS. 1 and 2 is directly connected to the memory controller101 via the buffers 105, i.e. not connected to the respective DRAMs 110.

As shown in FIG. 3, the data wiring 111, the command/address wiring 112,and the clock wiring (CLK/CLKB) 113 are connected from the memorycontroller 101 to the buffer 105 of the memory module 103 a, thenconnected therefrom to the buffer 105 of the memory module 103 b of thenext stage. Likewise, these wirings are connected to the buffers 105 ofthe subsequent memory modules 103 c and 103 d in order and terminated attheir termination ends with terminating resistances, thereby forming adaisy chain. In other words, the wirings such as the data wiring 111 areconnected to the buffers 105 between the memory controller 101 and thememory module 103 a, between the memory modules 103 a and 103 b, betweenthe memory modules 103 b and 103 c, and between the memory modules 103 cand 103 d, and further, connected point-to-point relative to the buffers105 of the prior and subsequent stages in cascade.

As shown in FIG. 2, the data wiring (DQ) 111, the command/address wiring(Cmd/Add) 112, and the clock wiring 113 can be respectively parceledinto wiring portions on the mother board and module wiring portions inthe memory modules. Further, in the shown memory system, moduleidentification wiring 114 for transmitting a module identifying signalMID that identifies one of the memory modules 103 a to 103 d is alsodisposed between the memory controller 101 and the buffer 105 andbetween the buffers 105.

As shown in FIG. 1, the buffer 105 in each memory module 103 and theDRAMs 110 mounted in the subject memory module 103 are connectedtogether via internal data wiring 111′, internal command/address wiring112′, and internal clock wiring 113′. Herein, the internal data wiring111′ is connected individually to the respective DRAMs 110 on the memorymodule 103, while the internal command/address wiring 112′ and theinternal clock wiring 113′ are respectively provided so as to be commonto the DRAMs 110 disposed on the left side or the right side of thebuffer 105.

Assuming that the shown DRAM 110 is a DRAM of a x-8 configuration thatcan write and read data per 8 bits, data transmission/reception isperformed on the unit of 8 bits between each DRAM 110 and the buffer 105in each memory module 103.

Description will be given about the shown memory system in more detail.Each of the memory modules 103 a and 103 b has eight DRAMs 110, whereinfour of them are placed on each of the left and right sides of thebuffer 105. Further, the data wiring 111 between the memory controller101 and the buffer 105 and between the buffers 105 has a 32-bit width.When either one of the memory modules 103 a and 103 b is selected by acommand/address signal and a module identifying signal MID, the eightDRAMs 110 on the selected memory module 103 a, for example, areactivated to thereby set the state wherein data of a 64-bit width intotal can be transmitted/received between the eight DRAMs 110 and thebuffer 105.

On the other hand, when a DRAM 110 denoted by a broken line in FIGS. 1and 2 is added to each memory module 103, four DRAMs 110 are arranged onthe left side of the buffer 105, while five DRAMs 110 are arranged onthe right side thereof and the data wiring 111 between the memorycontroller 101 and the buffer 105 and between the buffers 105 has a36-bit width. In this configuration, when either one of the memorymodules 103 a and 103 b is selected by a command/address signal and amodule identifying signal MID, the nine DRAMs 110 on the selected memorymodule 103 a, for example, are activated to thereby set the statewherein data of a 72-bit width in total can be transmitted/receivedbetween the nine DRAMs 110 and the buffer 105.

As described above, it is seen that the eight or nine DRAMs 110 on eachof the memory modules 103 a and 103 b form a simultaneously accessiblerank in the memory system shown in FIGS. 1 and 2.

Now, referring to FIGS. 1 and 2, further description will be given aboutthe wiring between the memory controller 101 and the memory module 103 aand the wiring between the adjacent memory modules 103. First, the datawiring 111 will be described. Although data of a 64-bit or 72-bit widthare transmitted/received via the internal wiring 111′ between the buffer105 and the DRAMs 110, the data wiring 111 between the memory controller101 and the buffer 105 and between the buffers 105 has a 32-bit width ora 36-bit width as shown in FIGS. 1 and 2.

This means that a data signal, multiplexed or compressed into packets,is transmitted/received on the data wiring 111 at a transmission speedhigher than a data frequency, i.e. an operation speed, of the DRAM 110.In the shown example, data is transferred on the data wiring 111 at aspeed that is n (n is a positive integer) times the operation speed ofthe DRAM 110. Accordingly, when compressed into packets, the number ofdata lines is reduced to about 1/n (not necessarily 1/n because there isactually an indivisible case or the like).

Like the data wiring 111, the command/address wiring 112 is connectedpoint-to-point between the memory controller 101 and the memory module103 and between the adjacent memory modules 103. In the command/addresswiring 112, a command/address signal is transferred at a speed that is m(m is a positive integer) times a command/address signal frequency ofthe DRAM 110 and, when compressed into packets, the number of signallines is reduced to about 1/m (also not necessarily 1/m because there isactually an indivisible case or the like).

The buffer 105 provided on each memory module 103 has a function ofreceiving a data signal or a command/address signal from the memorycontroller 101 or the memory module 103 of the prior stage and encodingpackets of the data or command/address signal to provide the number ofsignals corresponding to the DRAMs on the subject memory module 103.Further, the buffer 105 has a function of dividing a frequency of theencoded data or command/address signals into 1/n or 1/m times thefrequency and sending them to the DRAMs 110.

Furthermore, the buffer 105 also has a function of transferring ortransmitting a command/address signal to the cascade-connected memorymodule 103 of the next stage, a function of bidirectionallytransmitting/receiving a data signal relative to the next-stage memorymodule, and a function of identifying a module identifying signal MIDrepresenting a packet destination of a data or command/address signal.Inasmuch as the functions of dividing, identification, etc. in thebuffer 105 can be easily realized using the usual techniques, detailsthereof are not given here. In any case, the respective wirings on thememory modules 103 are connected with wiring layout having only suchbranching that can be electrically ignored.

Now, referring to FIG. 2, description will be given about transmissionspeeds in the respective wirings. First, it is assumed that the DRAMs110 on each memory module 103 are SDRAMs and employ the DDR (Double DataRate) system that performs input/output of data synchronously with bothleading and trailing edges of a clock. Further, assuming that aninternal clock frequency of 666 MHz is given to the internal clockwiring 113′ between the buffer 105 and the DRAMs 110 in each memorymodule 103, data is transferred in the internal data wiring 111′ at adata transmission speed of 1.33 Gbps, i.e. at a data frequency of 1.33GHz, and a command/address signal of 666 Mbps is supplied from thebuffer 105 in the internal command/address wiring 112′.

In this example, it is assumed that the clock wiring 113 disposed on themother board 100 is given, from the memory controller 101, clocks havinga clock frequency of 1.33 GHz that is twice the internal clockfrequency. As shown in FIG. 2, the data wiring 111 and thecommand/address wiring 112 are fed with data and command/address signalsat a transmission speed of 2.66 Gbps that is twice the clock frequency,while transmission speeds of the internal data wiring 111′ and theinternal command/address wiring 112′ are 1.33 Gbps and 666 Mbps,respectively. Therefore, it is seen that n=2 and m=4 in the shownexample.

As described above, by multiplexing the signals on the mother board toimplement high-frequency transmission, the number of the lines on themother board can be reduced. The data wiring 111 can be reduced to ½ byduplexing the signal, while the command/address wiring 112 can bereduced to ¼ by quadplexing the signal. Further, by duplexing data, amemory system with data wiring of a 32-bit width (or data wiring of a36-bit width) can be operated as a memory system of a 64-bit (or 72-bit)configuration.

The memory system shown in FIGS. 1 to 3 requires a layout configurationfor inputting/outputting a data signal of a 32-bit or 36-bit width fromthe module connector 104 (FIG. 3) to the buffer 105. As describedbefore, the internal data wiring 111′ and the internal clock wiring 113′and command/address wiring 112′ on the memory module 103 are connectedwith the wiring layout having only electrically ignorable branching.However, since the number of the DRAMs connected to the internal datawiring 111′ and the number of the DRAMs connected to the internal clockwiring 113′ and command/address wiring 112′ differ from each other, itcan be considered that a difference in signal propagation time caused bya difference in load may be a problem upon high-frequency operation.Further, as clear from FIGS. 1 and 2, inasmuch as the clocks and thecommand/address signals are given to all the DRAMs 110 on each memorymodule 103, the total input load is large, and therefore, it can beconsidered that a problem may be raised upon high-frequency operation.

Referring to FIGS. 4 and 5, a memory system according to a secondpreferred embodiment of the present invention has a configuration thatcan reduce the foregoing problems associated with the first preferredembodiment. The shown memory system differs from the memory systemaccording to the first preferred embodiment in that each of memorymodules 103 a to 103 d (FIG. 5) is provided with two buffers 105 a and105 b. Specifically, each of the buffers 105 a and 105 b of each of thememory modules 103 a and 103 b is connected to a plurality of DRAMs 110a arranged on both left and right sides thereof via internal data wiring(DQ) 111′, internal command/address wiring 112′, and internal clockwiring 113′.

In the shown example, the DRAMs 110 a within each memory module 103 areindividually connected to the buffer 105 a or 105 b via the internaldata wiring (DQ) 111′, and further, commonly connected to the left orright side of the buffer 105 a or 105 b via the internal command/addresswiring 112′ and the internal clock wiring 113′.

Further, like in the first embodiment, the buffers 105 a and 105 bwithin each memory module 103 are connected to a memory controller 101or the memory module of the next stage via data wirings 111,command/address wirings 112, and clock wirings 113. This configurationis the same as the connection relationship shown in FIG. 3 and, as aresult, the buffers 105 a and 105 b of each memory module 103 areconnected point-to-point to the buffers 105 a and 105 b of other memorymodules 103. That is, the data wirings 111, the command/address wirings112, and the clock wirings 113 are connected to the buffers 105 a and105 b of the subsequent stages in cascade in order, thereby formingdaisy chains.

In the example shown in FIG. 5, x-8 configuration DRAMs 110 a eachinputting/outputting data per 8 bits are mounted on each memory module103, and each DRAM 110 a performs an input/output operation according toclocks having a clock frequency of 666 MHz given via the internal clockwiring 113′. As a result a command/address signal and data aretransmitted in the internal command/address wiring 112′ and the internaldata wiring 111′ at transmission speeds of 666 MHz and 1.33 GHz,respectively.

On the other hand, the memory controller 101 and the buffers 105 a and105 b of the memory module 103 a are connected together via the datawirings 111, the command/address wirings 112, the clock wirings 113, andmodule identification wirings 114. Further, these wirings extend to thebuffers 105 a and 105 b of the memory module 103 b of the next stage,and are further connected to the buffers 105 a and 105 b of the memorymodules 103 c and 103 d that are shown behind the memory module 103 b inFIG. 5. In this manner, the data wirings 111, along with thecommand/address wirings 112 and the clock wirings 113, are connected tothe two buffers 105 a and 105 b concentrically, i.e. in groups.

In FIG. 5, clocks having a frequency of 1.33 GHz are given on the clockwirings 113, and a command/address signal and data areinputted/outputted on the command/address wirings 112 and the datawirings 111 at a transmission speed of 2.66 Gbps. Therefore, it is seenthat each of the buffers 105 a and 105 b can produce internal clocks,internal command/address signals, and internal data by converting theclocks, the command/address signal, and the data from the memorycontroller 101 into two or four parallel signals.

In this configuration, by simultaneously operating the buffers 105 a and105 b of each memory module 103, it is possible to construct a memorysystem that inputs/outputs data at a 32-bit or 36-bit width, like in thefirst preferred embodiment. In case of the memory system fortransmitting/receiving data of a 32-bit width, the two x-8 configurationDRAMs 110 a are placed on each side of each of the buffers 105 a and 105b. When each memory module 103 is selected, the eight DRAMs 110 a oneach memory module 103 are activated by both buffers 105 a and 105 b sothat data of a 64-bit width can be transmitted/received between thebuffers 105 a and 105 b and the eight DRAMs 110 a. In the shown example,the memory controller 101 and each of the buffers 105 a and 105 b areconnected by the data wiring 111 of a 16-bit width, and these datawirings 111 are also connected to the buffers of the memory modules ofthe subsequent stages. As clear from this, multiplexed data istransmitted on the data wirings 111 like in the first preferredembodiment.

On the other hand, in case of the memory system fortransmitting/receiving data of a 36-bit width, data of a 72-bit widthcan be transmitted/received between the nine DRAMs 110 a and the buffers105 a and 105 b on each memory module 103. In the example shown in FIG.5, data of a 40-bit width is transmitted/received between the buffer 105a and the five DRAMs 110 a disposed on both sides of the buffer 105 a,while data of a 32-bit width is transmitted/received between the buffer105 b and the four DRAMs 110 a disposed on both sides of the buffer 105b.

In this case, the data wiring 111 between the memory controller 101 andthe buffer 105 a has a 20-bit width, while the data wiring 111 betweenthe memory controller 101 and the buffer 105 b has a 16-bit width and,like in the first preferred embodiment, data and command/address signalsthat are multiplexed, i.e. compressed into packets, aretransmitted/received on the data wirings 111 and the command/addresswirings 112, respectively.

In the shown memory system, the number of the DRAMs 110 a driven by eachof the buffers 105 a and 105 b can be reduced to half as compared withthe first preferred embodiment, and therefore, the number of lines ineach of the buffers 105 a and 105 b on the memory module 103 can bereduced and the wiring length can be shortened. Further, inasmuch as thenumber of the DRAMs 110 a, forming loads, of each of the buffers 105 aand 105 b can be reduced, a difference in input load at the internaldata wiring 111′, and the internal command/address wiring 112′ and theinternal clock wiring 113′ can be reduced so that the memory systemsuitable for high-frequency operation can be constructed.

In the memory system shown in FIG. 4, in case of the memory system fortransmitting/receiving the data of the 36-bit width between the memorycontroller 101 and the buffers 110 a and 105 b, it is readily understoodthat the DRAM 110 a surrounded by a broken line in FIG. 4 is connectedin each memory module 103, which is also clear from FIG. 5.

The memory system according to the second preferred embodiment shown inFIGS. 4 and 5 can be applied with various modifications. For example,instead of the x-8 configuration DRAMs, x-4 configuration DRAMs eachinputting/outputting data per 4 bits, or x-16 configuration DRAMs eachtransmitting/receiving data per 16 bits may be disposed on both sides ofthe two buffers. Further, the present invention is applicable not onlyto a memory system having DRAMs arranged on only one side of a moduleboard of each memory module, but also to a memory system having DRAMsarranged on both front and back sides thereof. Moreover, the presentinvention is likewise applicable to a system in which a plurality ofDRAMs arranged on each memory module are classified into a plurality ofranks.

In the memory system according to the foregoing preferred embodiment, acommand/address signal given to each memory module is given individuallyto a plurality of buffers, and therefore, the number of command/addresssignal pins is increased by a multiple of the number of the buffers.However, inasmuch as the command/address signal is multiplexed, theincrease is not so large.

Referring to FIG. 6, there is shown one example of a memory systemaccording to a third preferred embodiment of the present invention. Theshown memory system has a configuration that can reduce the number ofinternal data lines between the module connector 104 (FIG. 3) and abuffer without increasing the number of buffers in each memory module.Specifically, the memory system shown in FIG. 6 comprises a memorycontroller 101 and a plurality of memory modules 103 (only 103 a and 103b are shown in the figure), wherein 16 DRAMs 110 (subscript omitted) aremounted on the front and back sides of each memory module 103. It isassumed that the shown DRAM 110 is a x-8 configuration DRAM thatperforms write/read per 8 bits. At the centers of the memory modules 103a and 103 b, buffers 105(11) and 105(21) are disposed, respectively. Thebuffer 105(11) is connected with 16-bit width data wiring (DQ) 111,command/address wiring (Cmd/Add) 112, clock wiring (CLK) 113, and moduleidentifying wiring (MID) 114, and the buffer 105(21) is likewiseconnected with 16-bit width data wiring (DQ) 11, command/address wiring(Cmd/Add) 112, clock wiring (CLK) 113, and module identifying wiring(MID) 114. These wirings of each of the buffers 105(11) and 105(21) areconnected to buffers of non-shown memory modules to thereby form a daisychain.

In this embodiment, the total 32 DRAMs 110 of the two memory modules 103a and 103 b are classified into four groups each including eight DRAMs,which operate as ranks 1 to 4. In this connection, wiring from thebuffer 105(11), 105(21) to the DRAMs 110 in the memory module 103 a, 103b is such that the wiring is common to the corresponding DRAMs 110 onthe front and back sides of the memory module 103 a, 103 b and connectedtogether through vias within the memory module 103 a, 103 b, andconnected to the same DQ terminal of the buffer 105(11), 105(21).Specifically, the DRAMs 110 used in the rank 1 and the rank 3 arelocated in corresponding positions on the front and back sides of eachof the memory modules 103 a and 103 b, while the DRAMs 10 used in therank 2 and the rank 4 are likewise located in corresponding positions onthe front and back sides of each of the memory modules 103 a and 103 b,and the DRAMs of the same rank are activated by the use of address bitsfor selecting the rank. Taking this into consideration, in FIG. 6, theDRAMs 110 belonging to the rank 1 are assigned a subscript r1 and,likewise, the DRAMs 110 of the ranks 2 to 4 are featured by r2 to r4.

In this configuration, in case of operating the DRAMs 110 of the rank 1,when the four DRAMs 110 r 1 of each of the memory modules 103 a and 103b are selected, the state is set wherein data of a 32-bit width istransmitted/received via internal data wiring 111′ between each of thebuffers 105(11) and 105(21) of the memory modules 103 a and 103 b andthe DRAMs 110 r 1. In this state, the buffers 105(11) and 105(21) arerespectively connected to the memory controller 101 via the data wirings111 each having a 16-bit width, and therefore, performtransmission/reception of data relative to the memory controller 101 as32-bit data wiring in total.

In this manner, the four ranks are formed by using the two memorymodules 103 a and 103 b as a pair, so that the wiring of the ranks 1 and3 can be made common and the wiring of the ranks 2 and 4 can be likewisemade common in each of the memory modules 103 a and 103 b to therebyreduce the number of lines in the memory modules 103 a and 103 b.

Here, the memory system shown in FIG. 6 differs from the memory systemaccording to the first preferred embodiment in that each of the buffers105(11) and 103(21) is directly connected to the memory controller 101,and further differs from the memory system according to the secondpreferred embodiment in that the single buffer 105(11), 105(21) of thememory module 103 a, 103 b is connected via the data wiring 111 of the16-bit width.

In the configuration shown in FIG. 6, a chip select signal (CS) is usedfor identifying the ranks 1 to 4. However, bits for identifying theranks 1 to 4 may be added separately.

Now, description will be given about an operation of the memory systemshown in FIG. 6. When one command/address signal is outputted from thememory controller 101, this command/address signal is, in this example,fed to the two memory modules 103 a and 103 b. In this event, it isneedless to say that the command/address signal is outputted from thememory controller 101 synchronously with the clocks. The command/addresssignal activates the eight DRAMs of the same rank in the two memorymodules 103 a and 103 b, for example, the DRAMs 110 r 1 of the rank 1,so that a write/read operation of data is implemented between theactivated eight DRAMs 110 r 1 and the buffers 105(11) and 105(21) in thememory modules 103 a and 103 b. In this case, the four DRAMs 110 r 1 onthe memory module 103 a are activated so that 32-bit width data can betransmitted/received relative to the buffer 105(11), while the fourDRAMs 110 r 1 on the memory module 103 b are activated so that 32-bitwidth data can be transmitted/received relative to the buffer 105(21)likewise.

Inasmuch as the buffers 105(11) and 105(21) are connected to the memorycontroller 101 via the 16-bit width data wirings 111, respectively,multiplexed data is transmitted between the memory controller 101 andthe buffers 105(11) and 105(21), which is the same as the foregoingpreferred embodiments.

The buffers 105(11) and 105(21) of the memory modules 103 a and 103 bmay be connected to buffers of non-shown other memory modules,respectively, to thereby form daisy chains. Therefore, the buffers ofthe shown memory system may be expressed by 105(12˜1k) and 105(22˜2k) (kis a positive integer equal to 3 or greater). As clear from this, memorymodules of the shown memory system may be increased if necessary.

In the memory system according to the third preferred embodiment shownin FIG. 6, if the same DRAMs 110 as those in the memory system accordingto the first preferred embodiment are provided, the number of ranks ofthe DRAMs 110 is increased from two to four. There is a merit in thisembodiment that since the wiring in each memory module can be madecommon by providing the rank configuration of the DRAMs in each memorymodule, the degree of freedom of layout on each memory module 103 can beenhanced, and further, the number of buffer chips can be reduced ascompared with the second preferred embodiment. Further, as shown in FIG.6, inasmuch as data from the memory controller 101 to the memory module103 b is given to the buffer 105(21) of the memory module 103 bdirectly, i.e. without passing through another buffer, a logic delay dueto the buffer can be reduced as compared with the memory systemsaccording to the first and second preferred embodiments in which data istransmitted/received via the two buffers 105.

Referring to FIG. 7, there is shown a modification of the memory systemaccording to the third preferred embodiment of the present invention.This memory system comprises only two memory modules 103 a and 103 b,and is a memory system that does not take into consideration theincrease of memory modules. In this example, buffers 105 respectivelyprovided in the memory modules 103 a and 103 b do not form a daisy chainrelative to other memory modules, but are terminated with terminatingresistances. In other words, in the shown example, inasmuch as there areno other memory modules that are connected in cascade, the buffers ofthe memory modules 103 a and 103 b are represented by reference numerals105(1) and 105(2), respectively, in FIG. 7. On the other hand, 16 DRAMs110 provided on the front and back sides of each of the memory modules103 a and 103 b are grouped into four ranks, and wiring of the rank 1and the rank 3 is made common and wiring of the rank 2 and the rank 4 islikewise made common in each of the memory modules 103 a and 103 b,which is the sane as FIG. 6.

Referring to FIG. 8, there is shown another modification of the memorysystem according to the third preferred embodiment of the presentinvention. This modification comprises four memory modules 103 a to 103d each having a single buffer 105, and the buffers 105(1) to 105(4)(buffers 105(3) and 105(4) are not shown) of these memory modules aredirectly connected to the memory controller 101, which differs from thememory systems of FIGS. 6 and 7. Accordingly, each buffer 105 of thememory system shown in FIG. 8 is connected to the memory controller 101with the number of data lines corresponding to a quarter of a 32-bitwidth, and x-8 configuration DRAMs 110 on each of the memory modules 103a to 103 d are classified into eight ranks, thereby improving the degreeof freedom of layout of each of the memory modules 103 a to 103 d.

As described above, in this embodiment, the 8-rank configuration isformed by using the four memory modules 103 a to 103 d as a set. The 16DRAMs 110 are mounted in each of the memory modules 103 a to 103 d,wherein the four DRAMs arranged on the right on the front side of eachmemory module are classified into the ranks 1 to 4, the four DRAMsarranged on the right on the back side of each memory module areclassified into the ranks 5 to 8, the four DRAMs arranged on the left onthe front side of each memory module are classified into the ranks 1 to4, and the four DRAMs arranged on the left on the back side of eachmemory module are classified into the ranks 5 to 8. The rank 1 and therank 5, the rank 2 and the rank 6, the rank 3 and the rank 7, and therank 4 and the rank 8 are located in corresponding positions on thefront and back sides of each memory module, and wiring from each of thebuffers 105(1) to 105(4) to the DRAMs of those ranks is made common andconnected through vias. The memory system shown in FIG. 8 differs fromthat shown in FIG. 6 in that each of data wirings to the memory modules103(a) to 103(d) is 8 bits in the memory system of FIG. 8 to therebyform 32-bit data wiring over the whole memory system.

As described before, the DRAMs 110 of each of the memory modules 103 aand 103 b are classified into the eight ranks and, for clarifying this,the DRAMs 110 of the ranks 1 to 8 are represented by reference symbols110 r 1 to 110 r 8, respectively, in FIG. 8.

In this configuration, when an address signal is given from the memorycontroller 101 as a command/address signal (Cmd/Add), the two DRAMs ofthe same rank in each of the memory modules 103 a to 103 d, for example,the two DRAMs 110 r 1 of the rank 1 in each memory module, areactivated, and therefore, the state is set wherein 16-bit width data canbe transmitted/received relative to each of the buffers 105(1) to 105(4)so that 64-bit width data in total can be transmitted/received over thefour buffers 105(1) to 105(4). As shown in the figure, data wiring 111of each of the memory modules 103 a to 103 d is 8 bits, and multiplexeddata is transmitted/received between the memory controller 101 and eachof the buffers 105(1) to 105(4) on the data line 111 of each of thememory modules 103 a to 103 d.

Referring to FIG. 9, there is shown still another modification of thememory system according to the third preferred embodiment of the presentinvention, wherein a 2-rank memory system is formed by using two memorymodules 103 a and 103 b as a pair. The memory system of FIG. 9 differsfrom that of FIG. 6 in that 16 DRAMs disposed on the front side of thetwo memory modules 103 a and 103 b form a rank 1, while 16 DRAMs on theback side thereof form a rank 2, and each DRAM 110 is a x-4configuration DRAM. Further, in FIG. 9, the eight DRAMs 110 mounted onthe front side of each of the memory modules 103 a and 103 b form therank 1, while the eight DRAMs 110 mounted on the back side thereof formthe rank 2. In this connection, in FIG. 9, the 16 DRAMs 110 belonging tothe rank 1 and arranged in the memory modules 103 a and 103 b aredenoted by reference symbol 110 r 1, while the 16 DRAMs 110 belonging tothe rank 2 are denoted by reference symbol 110 r 2. Further, the DRAMs110 r 1 and 110 r 2 of the ranks 1 and 2 arranged on the front and backsides of each of the memory modules 103 a and 103 b are commonlyconnected to each other via internal data wiring of a 4-bit width.

On the other hand, a buffer 105 of each of the memory modules 103 a and103 b is connected to the memory controller 101 via 16-bit width datawiring 111, and multiplexed data is transmitted on each of the datawirings 111, which is the same as the other examples. Even with thisconfiguration, like the memory system shown in FIG. 6, 32-bit width datais transmitted between the eight DRAMs 110 r 1, 110 r 2 of the memorymodule 103 a, 103 b and the buffer 105, and further, multiplexed data ofa 16-bit width is transmitted between each buffer 105 and the memorycontroller 101.

Referring to FIG. 10, there is shown an example having a 36-bit buswidth with parity bits, as still another modification of the memorysystem according to the third preferred embodiment of the presentinvention.

This example differs from the memory system shown in FIG. 9 in that ninex-4 configuration DRAMs 110 are mounted on each of the front and backsides of each of memory modules 103 a and 103 b, and data wiring 11between a buffer 105 of each of the memory modules 103 a and 103 b andthe memory controller 101 has an 18-bit width. Specifically, in each ofthe memory modules 103 a and 103 b shown in FIG. 10, four DRAMs 110 arearranged on the left on each of the front and back sides of the buffer105, while five DRAMs 110 are arranged on the right of each of the frontand back sides of the buffer 105. Here, it is assumed that the DRAM 110placed on the rightmost on each of the front and back sides of each ofthe memory modules 103 a and 103 b is used as a DRAM for parity.

Like FIG. 9, this example is also a 2-rank memory system using the twomemory modules 103 a and 103 b as a pair. Further, the 18 DRAMs 110arranged on the front side of the two memory modules 103 a and 103 bform a rank 1, while the 18 DRAMs 110 arranged on the back side thereofform a rank 2. In this connection, the DRAMs of the ranks 1 and 2 aredenoted by reference symbols 110 r 1 and 110 r 2, respectively. Further,internal data wiring of the DRAMs 110 r 1 and 110 r 2 of the ranks 1 and2 arranged on the front and back sides of each memory module is common,which is also the same as FIG. 9.

Further, the buffer 105 of each of the memory modules 103 a and 103 b isconnected to the memory controller 101 via data wiring 111 correspondingto an 18-bit width, and is connected to buffers of non-shown memorymodules in cascade to thereby form a daisy chain.

In this configuration, multiplexed data with parity istransmitted/received between the memory controller 101 and the memorymodule 103 a or 103 b.

Comparison will be made between the first and second preferredembodiments and the third preferred embodiment. In the first and secondpreferred embodiments, since transmission/reception of data between theDRAMs on the cascade-connected second memory module and the memorycontroller is carried out via two buffer chips, a logic delay necessaryfor reception/transmission processing at the buffer chips becomes twicethe third preferred embodiment. On the other hand, in the thirdpreferred embodiment, although there is the merit of reducing the numberof buffers to be passed through it is necessary to increase the numberof ranks of the DRAMs on the memory module.

Referring to FIG. 11, more detailed description will be given about thesignal transmission system between the memory controller (MC) 101 andeach memory module 103 in the foregoing memory systems. In the shownexample, it is assumed for simplifying the description that buffers 105a and 105 b of memory modules 103 a and 103 b are connected in cascade.In this system, the memory controller 101 transmits a command/addresssignal (CA) synchronously with a clock signal, and these command/addresssignal (CA) and the clock signal are received at the buffers 105 a and105 b of the memory modules 103 a and 103 b in order.

On the other hand, data (DQ) signals are transmitted/received at thebuffers 105 a and 105 b and the memory controller 101 synchronously witha plurality of pairs of bidirectional clock signals (complementary) CLKand CLKB. Specifically, when writing data into the DRAMs of the memorymodules 103 a and 103 b from the memory controller 101, the data istransmitted to the buffers 105 a and 105 b synchronously with clocksoutputted from the memory controller 101, while, when reading data fromthe DRAMs of the memory modules 103 a and 103 b, the buffers 105 a and105 b of the memory modules 103 a and 103 b produce clocks from internalclocks of the DRAMs and output read data from the DRAMs to the memorycontroller 101 synchronously with the produced clocks. Upon packettransmission of a command/address signal and a data signal, a moduleidentifying signal MID is transmitted from the memory controller 101simultaneously with these command/address signal and data signal, andthe buffers 105 a and 105 b identify effective head data of the signalsand a reception/transmission destination memory module using the signalMID.

Referring to FIG. 12, there is shown a timing relationship in the systemshown in FIG. 11. In the shown example, clocks having a frequency of1.33 GHz (i.e. period of 0.75 ns) are produced from the memorycontroller (MC) 101 (see the first line in FIG. 12) and, synchronouslywith leading and trailing edges of the clocks, data is transmitted fromthe memory controller (MC) 101 to the buffers (see the third line). As aresult, the data is transmitted to the buffers 105 a and 105 b from thememory controller (MC) 101 at a transmission speed of 2.66 Gbps.

On the other hand, internal clocks having a frequency of 666 MHz (periodof 1.5 ns) are produced from the buffers 105 a and 105 b relative to theDRAMs (see the second line) and, with a lapse of a buffer internallatency time, the data received at the buffers are written into theDRAMs at a transmission speed of 1.33 Gbps synchronously with leadingand trailing edges of the internal clocks (see the fourth line).

Then, synchronously with leading and trailing edges of the clocks havinga frequency of 1.33 GHz, a command/address signal (CA) is outputted tothe buffers 105 a and 105 b from the memory controller (MC) 101 (see thefifth line). After a lapse of a buffer internal latency time, thecommand/address signal (CA) is outputted to the DRAMs from the bufferssynchronously with leading edges of the internal clocks (see the sixthline). Therefore, the command/address signal is outputted from thememory controller (MC) to the buffers 105 a and 105 b at a transmissionspeed of 2.66 Gbps and outputted from the buffers to the DRAMs at atransmission speed of 666 Mbps. Further, a module identifying signal MIDis outputted from the memory controller (MC) to the buffers at atransmission speed of 2.66 Gbps synchronously with leading and trailingedges of the clocks of 1.336 Hz.

As clear from this, between the memory controller (MC) 101 and thebuffers 105 a and 105 b, the data is transferred at a frequency twicethe data frequency of the DRAMs, while the command/address signal (CA)is transferred at a four-times frequency. Therefore, the buffer on eachmemory module reduces the frequencies of the data and thecommand/address signal to ½ and ¼, respectively, by the use of afrequency divider or the like, and transmits them to the DRAMs.

Here, it is assumed that the memory system processes 8-bit continuousdata (burst). Specifically, it is assumed that 16-bit continuous data isoutputted on a 32-bit data bus at a transmission speed of 2.66 Gbps fromthe memory controller (MC) 101 to the buffers, and each buffer outputsthe 16-bit continuous data alternately to two DQ pins of the DRAMs as8-bit continuous data at a transmission speed of 1.33 Gbps.

Further, the command/address signal is outputted at a transmission speedof 2.66 Gbps from the memory controller (MC) to the buffers, and 4-bitdata, for example, of one command/address signal line is distributed tofour command/address signal lines at the buffer, thereby to be fed tothe DRAMs at a transmission speed of 666 Mbps.

Now, further detailed description will be given about the foregoingoperation by dividing it into data write and read operations, and acommand/address signal transfer operation. FIG. 13 shows a data writeoperation from the memory controller (MC) to the DRAMs. As describedabove, the memory controller (MC) 101 outputs the clocks of 1.33 GHz tothe buffers 105 (see the first line). Synchronously with the clocks, amodule identifying signal MID and data DQ0 m are outputted from thememory controller (MC) 101 (see the third and fourth lines).

Here, the module identifying signal MID includes an effective data headidentifying signal and a destination address, while the data DQ0 mincludes two data sequences DQ0 and DQ1 to be distributed to two DQ pinsof the DRAMs. Here, the data sequence DQ0 becomes continuous 8-bit dataDQ00, 10, 20, 30 . . . 70, while the data sequence DQ1 becomescontinuous 8-bit data DQ01, 11, 21, 31 . . . 71. As shown at the fourthline of FIG. 13, in the data DQ0 m, unit data of the data sequences DQ0and DQ1 are alternately placed synchronously with leading and trailingedges of the clocks shown at the first line. The data DQ0 m is outputtedfrom the memory controller (MC) 101 to the buffer 105 a synchronouslywith the clocks. Here, when the number of data lines from the memorycontroller (MC) to the buffer is 32 in total, since data is fed to twoDQ terminals of the DRAMs from the respective data lines, the system asa whole processes 8-bit continuous data at a 64-bit width. When thefirst-stage buffer 105 a judges from the module identifying signal MIDthat it is not addressed to the memory module 103 a to which the buffer105 a belongs, the module identifying signal MID is transferred to thenext-stage memory module 103 b along with the data DQ0 m (see the thirdand fourth lines).

Then, as shown at the second line, the buffer 105 a in the memory module103 a produces internal clocks of 666 MHz by dividing the clocks of 1.33GHz to half, and outputs them to the DRAMs. If the memory module 105 ais designated by the foregoing module identifying signal MID, the showndata DQ0 m is, after a lapse of a buffer latency, written into the givenDRAMs synchronously with the internal clocks. In the shown example, asshown at the fifth and sixth lines, the data sequences DQ0 and DQ1 areoutputted from the buffer 105 a to the two DRAMs synchronously withleading and trailing edges of the internal clocks.

Now, referring to FIG. 14, description will be given about an operationwhen the data DQ0 m is read from the DRAMs. In this case, it is assumedthat the data DQ0 m is read from the DRAMs of the memory module 103 a tothe memory controller (MC) 101 via the buffer 105 a. First, the buffer105 a is outputting the internal clocks of 666 MHz to the DRAMs (see thesecond line in FIG. 14), while outputting the clocks having a frequencyof 1.33 GHz to the memory controller (MC) 101 (see the first line). Inthis state, it is assumed that the data sequences DQ0 and DQ1 are readfrom two DQ terminals of the DRAMs. Here, it is assumed that the datasequences DQ0 and DQ1 include unit data D00, 10, 20 . . . 70 and unitdata D01, 11, 21 . . . 71, respectively (see the fifth and sixth lines).These unit data are sent out to the buffer 105 a from the two DQterminals synchronously with the internal clocks. The buffer 105 aoutputs a module identifying signal MID representing the memory module103 a to which the buffer 105 a belongs, to the memory controller (MC)as an effective data head identifying signal (see the third line).Subsequently, the buffer 105 a alternately combines the continuous 8-bitunit data of the data sequences DQ0 and DQ1 from the two DQ terminals tomultiplex them, and outputs the multiplexed data to the memorycontroller 101 synchronously with the clocks between the buffer 105 aand the memory controller 101 as the 16-bit read data DQ0 m. In case ofa buffer located at a later stage of the buffer 105 a like the buffer105 b, the data DQ0 m is given to the memory controller (MC) via thebuffer 105 a of the prior stage.

As described above, it is seen that the data transmission speed and theclock frequency between the memory controller (MC) 101 and the buffers105 a and 105 b are greater than the data transmission speed and theclock frequency between the buffers 105 a and 105 b and the DRAMs. Withthis configuration, the data write/read can be implemented at thetransmission speed depending on the operation speed of the DRAMs byreducing the number of lines between the memory controller (MC) 101 andthe buffers.

Further, referring to FIG. 15, there is shown an operation when acommand/address signal is given to the memory modules from the memorycontroller (MC) 101. As described before, it is assumed that the clockshaving a frequency of 1.33 GHz is fed to the buffers 105 a and 105 bfrom the memory controller (MC) 101 (see the first line), the internalclocks of 666 MHz are used between each buffer 105 and the DRAMs 110(see the second line). In this case, a module identifying signal MIDincludes a head identifying signal and a destination address signal of acommand/address signal CA0 m, the head identifying signal and thedestination address signal of the command/address signal CA0 m areoutputted from the memory controller (MC) 101 synchronously with leadingand trailing edges of the clocks of 1.33 GHz (see the third line), andthe signal MID is transferred to the buffer 105 a of the prior-stagememory module 103 a and also to the buffer 105 b of the subsequent-stagememory module 103 b.

In this example, simultaneously with the module identifying signal MID,address signals A0 to A3 are outputted from the memory controller (MC)101 to the buffer 105 a in multiplexed mode as the command/addresssignal CA0 m synchronously with leading and trailing edges of the clocksof 1.33 GHz, and subsequently, transferred to the buffer 105 b (see thefourth line). The buffer 105 of the memory module 103 designated by theforegoing module identifying signal MID feeds the address signals A0 toA3 to the DRAMs mounted on the designated memory module 103synchronously with the internal clocks. In FIG. 15, although only one ofcommand/address signals is shown, a plurality of command/address signalsgiven to the buffer are respectively converted into four command/addresssignals, for example, RAS, CAS, WE, and band address, and residualaddress signals etc. Through this, an operation mode, the DRAMs, andmemory cells in the DRAMs within the designated memory module areselected.

In the foregoing, the description was given mainly about the signaltransmission between the memory controller (MC) 101 and the memorymodules 103. However, it is desirable that signal transmission can beachieved at high speed also between each memory module 103 and the DRAMswithin the subject memory module 103.

For this purpose, the present invention proposes a method oftransmitting data at high speed between the buffer 105 and the DRAM.Hereinbelow, description will be given about a case wherein the datatransmission method according to the present invention is applied to thememory systems according to the foregoing first to third preferredembodiments of the present invention, but not necessarily limitedthereto.

Referring to FIG. 16, there are shown the DRAM 110 and the buffer 105 inthe memory module 103 of the foregoing memory system.

In FIG. 16, the DRAM 110 performs data reception/transmission relativeto the buffer 105 using data strobe signals DQS (and DQS* ascomplementary) (hereinbelow, only DQS will be described). In this case,the data strobe signal DQS is produced synchronously with clocks and,when bidirectionally transmitting data DQ, the data strobe signal DQS istransmitted in a transmission direction of the data DQ. For example,when transmitting the data DQ in a direction from the DRAM 110 to thebuffer 105, the data strobe signal DQS is also outputted from the DRAM110 to the buffer 105. This also applies to a case wherein data istransmitted from the buffer 105 to the DRAM 110.

Referring to FIG. 17A, there is shown an operation when writing datainto the DRAM 110 from the buffer 105 in FIG. 16, while FIG. 17B showsan operation when reading data from the DRAM 110. First, as shown inFIG. 17A, in case of data writing, after a write command (WRT) and anaddress (Add) are given to the DRAM from the buffer, data writing isimplemented with a data strobe signal DQS synchronously with leading andtrailing edges of the clocks, and this writing operation continues whilethe strobe signal DQS is given. Therefore, subsequently to theproduction of the command/address signal, data is written after a lapseof a predetermined latency time (WL=4 in the figure).

Further, as shown in FIG. 17B, also in case of data reading, a readcommand (RED) and an address (Add) are given to the DRAM from thebuffer, and data reading is implemented with a data strobe signal DQSsynchronously with leading and trailing edges of the clocks.

As described above, when the data strobe signal DQS is used, data istransmitted at the timing matched with the data strobe signal DQS, andreceived by the data strobe signal DQS. Accordingly, in thetransmission/reception system using the data strobe signal, it isnecessary that logics and layout delays of the data strobe signal DQSand the data DQ be matched with each other within the reception-sidedevice. However, when a delay changes due to temperature variation orvoltage variation, a setup and a hold time of a signal receivable by thedevice are deteriorated. For higher frequency operation, a shorter setupand hold time are required. Therefore, there is a limitation in speedupin the system wherein the data strobe signal is transmittedbidirectionally.

For carrying out data transmission/reception between the DRAM 110 andthe buffer 105 at higher speed, the present invention proposes to use,instead of the foregoing data strobe signal DQS, a signal (herein called“data phase signal DPS”) that is constantly transmitted bidirectionallyat the timing of a data signal and transmitted/received at the DRAM 110and the buffer 105. By using the data phase signal DPS that istransmitted/received bidirectionally, transmission/reception clocks canbe reproduced using a DLL in each device. Further, when the DLL is used,it is possible to cancel temperature variation or voltage variation by areplica delay, and further, since clocks can be set to the optimumtiming, data reception is made possible without using a delay logic.Therefore, a shorter setup and hold time can be achieved.

Referring to FIG. 18, there is shown a schematic configuration of a datatransmission system in which data transmission is performed between theDRAM 110 and the buffer 105 using the foregoing data phase signal DPS.As clear from comparison with FIG. 16, in the data transmission systemshown in FIG. 18, the data phase signal DPS is, instead of the datastrobe signal DQS, transmitted/received bidirectionally between thebuffer 105 and the DRAM 110, and the data phase signal DPS is, as atiming signal of data DQ transmitted from the buffer 105 or the DRAM110, fed to the other device. Specifically, when writing data DQ intothe DRAM 110 from the buffer 105, a write data phase signal DPS is fedto the DRAM 110 from the buffer 105 along with write data DQ atpredetermined write timing, while, when reading data DQ from the DRAM110, a read data phase signal DPS produced at timing different from theforegoing write timing is fed to the buffer 105 from the DRAM 110 alongwith read data DQ.

By identifying the write timing and the read timing, the DRAM 110 andthe buffer 105 respectively extract the write data phase signal and theread data phase signal (DPS), and perform writing and reading of thedata DQ using the extracted write data phase signal and read data phasesignal (DPS). As clear from this, the buffer 105 and the DRAM 110 areprovided with, in addition to the foregoing DLLs, circuits foridentifying the timings of the write data phase signal and the read dataphase signal (DPS).

Referring to FIG. 19, there are shown driver circuits and receivercircuits (i.e. transmission/reception circuits) of a buffer 105 and aDRAM 110 that are used when transmitting/receiving a data phase signalDPS between the buffer 105 and the DRAM 110 in a 1-rank configuration.As shown in the figure, each of the drivers of the buffer 105 and theDRAM 110 is provided with an open-drain N-channel MOS transistor. Thedrain of the N-channel MOS transistor of the DRAM 110 is connected witha variable resistance as a terminating resistance, while the drain ofthe N-channel MOS transistor of the buffer 105 is connected with a fixedresistance as a terminating resistance. When the variable resistance isconnected, a resistance value can be adjusted by the rank configurationof the DRAM side. Although the terminating resistance is provided withineach of the DRAM 110 and the buffer 105, it is readily understood thatit may be provided outside the device. A signal line for data phasesignal DPS transmission connected to the drains of both transistors ofthe DRAM 110 and the buffer 105 is connected to internal circuits of theDRAM 110 and the buffer 105 via amplifiers, respectively.

In the configuration shown in FIG. 19, a timing signal is given to thegate of the N-channel MOS transistor of the buffer 105 at predeterminedtiming and period to thereby turn ON/OFF the N-channel MOS transistor ofthe buffer 105, so that a write data phase signal DPS is fed to the DRAM110 from the buffer 105 and also to the inside of the buffer 105. On theother hand, a timing signal having a phase different from that of thetiming signal of the buffer 105 while produced at the same period isgiven to the gate of the N-channel MOS transistor of the DRAM 110 tothereby turn ON/OFF the N-channel MOS transistor of the DRAM 110, sothat a read data phase signal DPS is fed to the buffer 105 from the DRAM110 and also to the inside of the DRAM 110. As shown in the figure,since the driver in each of the DRAM 110 and the buffer 105 is inopen-drain mode, a bus is in a so-called wired OR configuration, andfurther, since the data phase signals DPS from the DRAM 110 and thebuffer 105 are outputted at the different timings, even if both signalsare outputted on the same signal line, there is no possibility ofcollision therebetween.

Referring to FIG. 20, there are shown driver circuits for data phasesignal DPS transmission/reception in a case wherein two DRAMs 110 in a2-rank configuration are connected to a buffer 105. As clear from thefigure, the configuration of FIG. 20 differs from that of FIG. 19 inthat the drivers of the two DRAMs 110 are connected to a single signalline of data phase signal DPS, while the configuration in each DRAM 110is the same. A variable resistance is connected to the drain of anN-channel MOS transistor in each DRAM 110 and, in this example, isadjusted to a resistance value suitable for the 2-rank configuration ofthe DRAMs 110.

Referring to FIGS. 21A and 21B along with FIG. 18, description will begiven about an operation when writing data DQ (i.e. write operation)relative to the DRAM 110, and an operation when reading data DQ (i.e.read operation) from the DRAM 110. As shown in FIG. 21A, upon writeoperation, the buffer 105 feeds a write command (WRT) and an addresssignal (Add) to the DRAM 110 synchronously with clocks. In this event awrite data phase signal WDPS is transmitted to the DRAM 110 from thebuffer 105 as a data phase signal DPS (see the fourth line). The shownwrite data phase signal WDPS is featured by the timing of a leading edge(rise) of each of pulses in a pulse stream having a frequency that is ¼times the clocks.

On the other hand, a read data phase signal RDPS is transmitted on thesame signal line in multiplexed mode from the DRAM 110 to the buffer 105at the timing that avoids collision with the write data phase signalWDPS (here, the timing shifted by two clocks). As shown at the fourthline in FIG. 21B, like the write data phase signal WDPS, the read dataphase signal RDPS is featured by the timings of leading edges (rise) ofa pulse stream having a frequency ¼ times the clocks, and the timingsthereof occur between the timings of the write data phase signal WDPS.In this manner, by deviating the timing between the write data phasesignal WDPS and the read data phase signal RDPS, both signals areprevented from collision therebetween on the single signal line. In theshown example, the timing between the write data phase signal WDPS andthe read data phase signal RDPS is shifted by two clocks. However, it isneedless to say that the timing is not limited thereto as long ascollision of both signals can be avoided.

Referring further to FIG. 21A, the phases of the clocks and the writedata phase signal (WDPS) agree with each other at the buffer 105 in thewrite operation from the buffer 105, while the phase of the read dataphase signal (RDPS) transmitted from the DRAM does not agree therewith.The data DQ is written after a lapse of a write latency time (WL=4) suchthat edges of rise (leading edge) and fall (trailing edge) of the clocksare placed at the center of a signal effective width.

Upon the read operation shown in FIG. 21B, the DRAM 110 reproduces theclocks in the DRAM 110 from the read data phase signal (RDPS). Matchingwith the timing of the reproduced clocks, the data DQ is transmitted tothe buffer 105 from the DRAM 110. In the shown example, the timing ofthe data coincides with the clock edge. However, the center of theeffective width may be matched with the clock edge.

In the foregoing example, the DRAM 110 and the buffer 105 constantlytransmit the data phase signals DPS bidirectionally on the same signalline during a normal operation, i.e. during an operation other than apower save mode. Further, the drivers of the DRAM 110 and the buffer 105are operated at the timings shifted by two clocks and, as shown in FIGS.19 and 20, the open-drain mode is employed. Accordingly, the bus is inthe so-called wired OR configuration, and therefore, there is nopossibility of bus fight.

Referring to FIGS. 21A and 21B, the description was given about thetiming relationship between the clocks and the write and read data phasesignals WDPS and RDPS upon writing and reading, and the timingrelationship among the data, the clocks, and the data phase signals(WDPS, RDPS). In the DRAM 110 and the buffer 105 having received thedata phase signals (WDPS, RDPS), it is necessary to reproduce thereinthe data transmission/reception clocks from the data phase signals(WDPS, RDPS).

Now, referring to FIG. 22, description will be given about a procedureof reproducing the data reception/transmission clocks inside the DRAM110 and the buffer 105 from the data phase signal DPS (write or readdata phase signal WDPS, RDPS) according to the present invention uponthe start of operation of the memory system.

First, the buffer 105 is transmitting clocks to the DRAM 110 (see thefirst line). In this example, the buffer 105 produces the clocks havinga frequency of 666 MHz. In this state, the buffer 105 transmits a writedata phase signal WDPS (see the second line) synchronously with theclocks. The shown write data phase signal WDPS is produced by dividingthe frequency of the clocks to quarter, and therefore, the write dataphase signal WDPS has a frequency of 666/4 MHz (i.e. ¼ times theclocks), and the write data phase signal WDPS is inputted into the DRAM110 with a time delay (see the third line).

The DRAM 110 produces, using a DLL provided in the inside thereof,internal clocks as reproduced clocks for determining data (DQ) receptiontiming, from the write data phase signal WDPS (see the fourth line). Theshown internal clocks have a frequency of 666 MHz.

Further, as shown in FIG. 22, after reproducing the data (DQ) receptionclocks as the internal clocks, the DRAM 110 produces a read data phasesignal RDPS shown by a solid line, based on the write data phase signalWDPS and the internal clocks by shifting the internal clocks by twoclocks, and transmits the read data phase signal RDPS to the buffer 105(see the fifth line). As shown in FIG. 22, the read data phase signalRDPS has a frequency ¼ times the internal clocks, and is produced so asnot to collide with the write data phase signal WDPS shown by a brokenline.

The read data phase signal RDPS is received at the buffer 105 with atime delay (see the sixth line), and the buffer 105 reproduces data (DQ)reception clocks of 666 MHz for receiving data from the DRAM 110 in thebuffer 105 (see the seventh line), from the received read data phasesignal RDPS. The timing chart shown in FIG. 22 conceptually explains thetiming relationship between the data phase signals DPS and the clocks,while, actually, as described later, the DRAM internal clocks for datareception and data output are produced at the optimum internal timings,respectively. Further, the shown clocks do not necessarily have a period¼ times that of the data phase signals DPS, and may be multiphaseclocks.

In any case, a feature of the shown transmission system resides in thatthe reception/transmission clocks within the DRAM 110 and the buffer 105are reproduced from the data phase signals WDPS and RDPS.

Referring to FIG. 23, description will be given about a concreteconfiguration of the DRAM 110 that performs the foregoing operation. Inthe figure, only an interface for transmitting/receiving data phasesignals DPS and data (DQ) relative to the buffer 105 is shown, and amemory cell region for writing and reading the data (DQ) is omitted inFIG. 23. Incidentally, the memory cell region of the DRAM 110 isconnected to a data (DQ) output driver 201 and a data receiver 202 tothereby perform reading and writing of the data (DQ). Further, the shownDRAM 110 is provided with a clock reproduction phase adjusting andfrequency multiplier circuit 205 composed of a DLL. A write data phasesignal WDPS is inputted into the DLL 205, while a read data phase signalRDPS from the DLL 205 is outputted via a DPS output driver 207. As clearfrom this, it is assumed that the shown DILL 205 is provided with adelay line including a plurality of delay cells, a phase detector, anintegrator, and a frequency multiplier.

Specifically, the DLL 205 is given data phase signals DPS includingwrite and read data phase signals WDPS and RDPS, and the data phasesignals DPS are also given to a reception phase comparing circuit 206and an output phase comparing circuit 209. The DLL 205 reproduces datareception DRAM internal clocks from the write data phase signal WDPS,and produces data reception feedback clocks. The data reception DRAMinternal clocks are given to the data receiver 202 so as to be used forwriting data DQ, while the data reception feedback clocks are given to areception replica 208 where the clocks are divided to quarter infrequency, so that a replica signal of the received write data phasesignal WDPS is outputted to the reception phase comparing circuit 206.The reception phase comparing circuit 206 suppresses the read data phasesignal RDPS by the replica signal from the reception replica 208 tothereby output to the DLL 205 a reception phase adjusting signalrelative to DPS output DRAM internal clocks with respect to only thewrite data phase signal WDPS.

Further, the shown DLL 205 delays the data reception DRAM internalclocks by two clocks to thereby output DRAM internal clocks foroutputting the read data phase signal RDPS, data output feedback clocks,and data output DRAM internal clocks. Among them, the DPS output DRAMinternal clocks are given to the DPS output driver 207 and the outputphase comparing circuit 209, while the data output DRAM internal clocksare fed to the data output driver 201. Further, the data output feedbackclocks are given to an output replica 210, and the output replica 210outputs a replica signal of the read data phase signal RDPS to theoutput phase comparing circuit 209. The DPS output driver 207 sends outthe read data phase signal RDPS to the buffer 105 in response to the DPSoutput DRAM internal clocks.

While suppressing the timing of the write data phase signal WDPS by theread replica signal given from the output replica 210, the output phasecomparing circuit 209 compares phases of the read data phase signal RDPSand the output of the DLL 205 and outputs to the DLL 205 an output phaseadjusting signal depending on a comparison result. As a result, the readdata phase signal RDPS is transmitted from the shown DRAM 110 to thebuffer 105.

As described above, in the shown DRAM 110, when the DRAM 110 transmitsthe read data phase signal RDPS, the DPS output DRAM internal clocks areoutputted so as not to perform phase comparison and, when receiving thewrite data phase signal WDPS, the DPS output DRAM internal clocks areinputted into the reception phase comparing circuit 206 to therebyperform an operation to inhibit feedback of a comparison value to theDLL 205.

Referring to FIG. 24, description will be given about a concreteconfiguration of the buffer 105 that performs datatransmission/reception relative to the DRAM 110 shown in FIG. 23. Likethe DRAM 110 shown in FIG. 23, the buffer 105 is provided with a DQoutput driver 301 for outputting data to the DRAM 110, and a datareceiver 302 for receiving read data from the DRAM 110, and furtherprovided with a DLL 305 forming a clock reproduction phase adjusting andfrequency multiplier circuit for data phase signal DPStransmission/reception. Further, in the shown buffer 105, DPS outputbuffer internal clocks are produced by a non-shown clock generator, andfed to a DPS output driver 307 and a reception phase comparing circuit306. The DPS output driver 307 divides the given clocks to quarter infrequency to thereby output a write data phase signal DPS (i.e. WDPS) tothe DRAM 110, and the write data phase signal WDPS is also given to theDLL 305 and the reception phase comparing circuit 306 within the buffer105.

In this state, when the read data phase signal RDPS is received from theDRAM 110, the DLL 305 of the buffer 105 produces data reception bufferinternal clocks and data reception feedback clocks, and outputs them tothe data receiver 302 and a reception replica 308, respectively. Thereception replica 308 produces a replica signal of the read datafeedback signal RDPS from the data reception feedback clocks, andoutputs it to the reception phase comparing circuit 306. As a result,the reception phase comparing circuit 306 ignores the write data phasesignal WDPS outputted from the DPS output driver 307, and outputs to theDLL 305 a reception phase adjusting signal with respect to a phase ofthe read data phase signal RDPS.

In the shown buffer 105, for reproducing the clocks from the read dataphase signal RDPS from the DRAM 110, the DPS output buffer internalclock signal is inputted into the reception phase comparing circuit 306to thereby inhibit feedback of a comparison value to the DLL.

FIG. 25 shows a timing chart upon the start of operation in the DRAM 110shown in FIG. 23, and FIG. 26 shows a timing chart during a normaloperation of the DRAM 110. Upon the start of operation shown in FIG. 25,a read data phase signal RDPS is not outputted to the buffer 105 fromthe DRAM 110. In FIG. 25, like in FIG. 22, DPS output buffer internalclocks of 666 MHz are produced in the buffer 105, and are divided toquarter in frequency at the DPS output driver 307 (FIG. 24) so that awrite data phase signal WDPS is outputted synchronously with the clocks(see the second line in FIG. 25). The write data phase signal WDPS isinputted into the DRAM 110 with a time delay (see the third line). Inthe DRAM 110, data reception feedback clocks having an advanced phaserelative to the received WDPS are produced at the DLL 205 (see thefourth line) and outputted to the reception replica 208, and a replicasignal of the WDPS is outputted from the reception replica 208 to thereception phase comparing circuit 206 (see the fifth line).

Following a reception phase adjusting signal from the reception phasecomparing circuit 206 and the received WDPS, the DLL 205 of the DRAM 110outputs data reception DRAM internal clocks to the data receiver 202(see the sixth line). Further, the DLL 205 of the DRAM 110 outputs tothe output replica 210 data output feedback clocks having an advancedphase relative to the internal clocks (see the seventh line), andoutputs to the DQ output driver 201 data output DRAM internal clockssynchronously with the data output feedback clocks (see the ninth line).Further, as shown at the eighth line in FIG. 25, a data output feedbacksignal is fed to the output phase comparing circuit 209 as a replicasignal from the output replica 210, and phase comparison is performedwith the presence of this replica signal so that DPS output DRAMinternal clocks as shown at the tenth line is outputted to the DPSoutput driver 207.

Now, referring to FIG. 26, the normal operation of the DRAM 110 shown inFIG. 23 will be described. In this case, as shown at the second andthird lines in FIG. 26, a write data phase signal WDPS is outputted fromthe buffer 105, while a read data phase signal RDPS (see thick lines) isoutputted from the DRAM 110. In this case, at the buffer 105, DPS outputclocks are produced, and the write data phase signal WDPS synchronouswith the DPS output clocks is transmitted to the DRAM 110, while, at theDRAM 110, data reception feedback clocks, a replica signal of the datareception feedback clocks, data reception DRAM internal clocks, dataoutput feedback clocks, and data output DRAM internal clocks areproduced, which is the same as FIG. 25 (see the fourth to eighth lines).Further, as shown at the ninth line, when the data output DRAM internalclocks are produced, the DLL 205 produces DPS output DRAM internalclocks by delaying the internal clocks by two clocks and, according tothe DPS output DRAM internal clocks, a read data phase signal RDPS isproduced from the DPS output driver 207 as shown by thick lines at thetenth line, and is received at the buffer 105 as shown at the secondline.

FIG. 27 shows a timing chart in the buffer 105 (FIG. 24) when theforegoing RDPS is received. It is assumed that data transmitted from theDRAM 110 is matched in phase with edges of the read data phase signalRDPS in this embodiment. In this connection, the buffer 105 shifts aphase of the reception buffer internal clocks by ¼ relative to a phaseof a replica signal from the reception replica 308 which is obtainedfrom the data reception feedback clocks.

In the foregoing examples, there have been shown the systems whereinwhen the internal clock signals are reproduced from the data phasesignals, the clocks are reproduced directly from the data phase signals.

Referring to FIGS. 28 and 29, there are shown modifications of the DRAM110 and the buffer 105 respectively shown in FIGS. 23 and 24. The DRAM110 shown in FIG. 28 differs from the DRAM 110 shown in FIG. 23 in thatclocks CLK are given to a DLL 205 from the exterior, and a data phasesignal DPS is not given to the DLL 205. In this connection, the shownDLL 205 not only operates as a clock reproduction phase adjustingcircuit, but also operates as a frequency divider for dividing afrequency of clocks. In this configuration, it is seen that, upon clockreproduction, the external clock signal CLK is fed to the DLL 205 as aclock source, only a phase of the signal CLK is adjusted at the DLL 205.In this manner, by giving the external clocks CLK to the DLL 205 andadjusting the phase of the clocks by the DLL 205, it is also possible toreproduce data reception DRAM internal clocks and data receptionfeedback clocks from the received write data phase signal WDPS, andfurther possible to produce DPS output DRAM internal clocks to therebytransmit a read data phase signal RDPS to the buffer 105.

The buffer 105 shown in FIG. 29 also differs from the buffer 105 shownin FIG. 24 in that a buffer internal clock signal is given to a DLL 305that operates as a clock phase adjusting circuit. When the buffer 105having the configuration shown in FIG. 29 is used, the DLL 305 adjusts aphase of clocks according to a reception phase adjusting signal from areception phase comparing circuit 306 to thereby produce data receptionbuffer internal clocks and data reception feedback clocks.

Referring to FIG. 30, description will be given about operations of thebuffer 105 and the DRAM 110 shown in FIGS. 28 and 29. In this example,an operation in the initial state of the DRAM 110 is shown wherein theDRAM 110 does not output the read data phase signal RDPS. As comparedwith FIG. 25, the example shown in FIG. 30 differs therefrom in thatexternal clocks of 666 MHz are produced in the DRAM 110 like in thebuffer 105 (see the third line). The other operations are the same asthose in FIG. 25 except that the operations are performed referring tosuch external clocks, and therefore, description thereof is omittedherein.

Referring to FIGS. 31 to 33, description will be given about anotherexample of a transmission system between the buffer 105 and the DRAM 110in the memory system according to the present invention. In theforegoing example the description has been given about the case whereinthe data phase signals DPS are outputted bidirectionally from the buffer105 and the DRAM 110 as the write and read data phase signals WDPS andRDPS. In FIG. 31, it is seen that a write data phase signal WDPS and aread data phase signal RDPS are outputted onto different signal linesfrom the buffer 105 and the DRAM 110. Other clocks (CLK),command/address (Cmd/Add), and data DQ are the same as those in FIG. 18.By employing this configuration, it is not necessary to multiplex thetwo data phase signals WDPS and RDPS onto the single signal line, sothat the configuration of the DLL used in each of the buffer 105 and theDRAM 110 can be simplified.

Referring to FIG. 32, description will be given about an operation, upondata writing, of the DRAM 110 shown in FIG. 31. In this case, a writecommand WRT and an address (Add) are outputted to the DRAM 110 from thebuffer 105 synchronously with clocks. In this event, a write data phasesignal WDPS is transmitted to the DRAM 110 from the buffer 105 whilebeing obtained by dividing the clocks CLK to quarter in frequency (seethe fourth line in FIG. 32). In the DRAM 110, according to internalclocks produced by using the write data phase signal WDPS as areference, data DQ is written into the DRAM 110 after a lapse of apredetermined latency time (see the fifth line).

On the other hand, in the DRAM 110, a read data phase signal RDPS isoutputted onto a signal line different from that for the write dataphase signal WDPS, at timing different from the reception timing of thewrite data phase signal WDPS.

As shown in FIG. 33, when a read command (RED) and an address (Add) arereceived at the DRAM 110, the DRAM 110 outputs read data DQ (see thefifth line) to the buffer 105 according to internal clocks (see thefirst line) produced based on a read data phase signal RDPS (see thefourth line). As clear from the figure, the output timing of the readdata phase signal RDPS differs from the reception timing of the writedata phase signal WDPS. In this example, the write data phase signalWDPS and the read data phase signal RDPS are shifted in phase by twoclocks therebetween for avoiding output noise such as mutualinterference or crosstalk therebetween.

Now, referring to FIGS. 34 and 35, description will be given aboutconcrete examples of the DRAM 110 and the buffer 105 shown in FIG. 31.When comparing the DRAM 110 shown in FIG. 34 and the DRAM 110 shown inFIG. 23, the DRAM 110 of FIG. 34 differs from the DRAM 110 of FIG. 23 inthat the write data phase signal WDPS and the read data phase signalRDPS are inputted thereinto via mutually different signal lines. In thisconnection, a read data phase signal output driver 207′ is connected tothe read data phase signal RDPS transmission signal line, butdisconnected from a DLL 205 of the DRAM 110 and the signal line of thewrite data phase signal WDPS, which differs from FIG. 23. The othercomponents are the same as those in FIG. 23.

Further, the buffer 105 shown in FIG. 35 differs from the buffer 105shown in FIG. 24 in that a write data phase signal WDPS transmissiondriver 307′ is connected to the write data phase signal transmissionsignal line, but disconnected from the read data phase signal RDPSreception signal line and a DLL 305 of the buffer 105. The othercomponents are the same as those in FIG. 24.

Here, a timing relationship between the DRAM 110 and the buffer 105shown in FIGS. 34 and 35 will be schematically described with referenceto FIG. 36. First, as shown in FIG. 36, the buffer 105 produces clockshaving a frequency of 666 MHz (see the first line), and divides theproduced clocks to quarter in frequency to thereby output a write dataphase signal WDPS onto the write data phase signal line (see the secondline). As shown at the third line, the write data phase signal WDPS isreceived at the DRAM 110 with a time delay. The DRAM 110 increases thereceived write data phase signal WDPS four times in frequency to therebyproduce internal clocks having a frequency of 666 MHz (see the fourthline), then shifts the produced internal clocks by two clocks anddivides them to quarter in frequency to thereby output a read data phasesignal RDPS as shown at the fifth line onto the read data phase signalline. The read data phase signal RDPS is received at the buffer 105 atthe timing shown at the sixth line, and the buffer 105 produces datareception internal clocks from the received read data phase signal RDPSas shown at the seventh line.

Referring also to FIG. 37, further detailed description will be givenabout an operation, during a normal time, of the DRAM 110 shown in FIG.34. Since operations upon the start are the same in the DRAM 110 of FIG.34 and the DRAM 110 of FIG. 23, description thereof is omitted. The DRAM110 shown in FIG. 34 is given the write data phase signal WDPS from thebuffer 105 via the write data phase signal line (see the third line inFIG. 37), and the write data phase signal WDPS is received at the DLL205, the reception phase comparing circuit 206, and the output phasecomparing circuit 209 in FIG. 34. As a result, the reception phasecomparing circuit 206 and the output phase comparing circuit 209 aregiven write data phase signals WDPS as shown at the fifth and eighthlines in FIG. 37 as input signals, respectively.

The DLL 205 refers also to a reception phase adjusting signal and anoutput phase adjusting signal from the reception phase comparing circuit206 and the output phase comparing circuit 209 to thereby output datareception feedback clocks shown at the fourth line and data receptionDRAM internal clocks shown at the sixth line in FIG. 37 to the receptionreplica 208 and the data receiver 202, respectively.

Further, the DLL 205 feeds data output feedback clocks and data outputDRAM internal clocks shown at the seventh and ninth lines to the outputreplica 210 and the DQ output driver 201, respectively. Of them, thedata output DRAM internal clocks are divided to quarter in frequency atthe DLL 205 and, as shown at the tenth line, fed to the RDPS outputdriver 207′ as RDPS output DRAM internal clocks. From the output driver207′, a read data phase signal RDPS shown at the eleventh line isoutputted to the buffer 105.

Referring to FIGS. 35 and 38, description will be given about anoperation of the buffer 105 upon read data reception. By the use of WDPSoutput buffer internal clocks (see the third line), a write data phasesignal WDPS is outputted onto the corresponding signal line (see thesecond line), and the read data phase signal RDPS is given to the DLL305 and the reception phase comparing circuit 306 in the buffer 105 viathe read data phase signal line (see the fifth line). The DLL 305 refersto a reception phase adjusting signal from the reception phase comparingcircuit 306 to thereby feed data reception feedback clocks and datareception buffer internal clocks shown at the fourth and sixth lines tothe reception replica 308 and the data receiver 302. Here, the showndata reception buffer internal clocks are shifted by ¼ phase relative tothe read data phase signal RDPS.

Referring to FIGS. 39 and 40, description will be given about otherexamples of a DRAM 110 and a buffer 105 that can realize thetransmission system shown in FIG. 31. The DRAM 110 shown in FIG. 39differs from the DRAM 110 shown in FIG. 34 in that clocks CLK are givenfrom the exterior like in FIG. 28. On the other hand, the buffer 105shown in FIG. 40 differs from the buffer 105 shown in FIG. 35 in that abuffer internal clock signal is given to a DLL 305 in the buffer 105. InFIG. 39, the external clocks are given to a DLL 205 in the DRAM 110,while a write data phase signal WDPS is fed to a reception phasecomparing circuit 206 and an output phase comparing circuit 209. Withthis configuration, an operation like that in FIG. 34 can also berealized.

In the buffer 105 shown in FIG. 40, a read data phase signal RDPS fromthe DRAM 110 is given to a reception phase comparing circuit 306, andthe DLL 305 produces data reception feedback clocks and data receptionbuffer internal clocks according to a reception phase adjusting signalfrom the reception phase comparing circuit 306 and the buffer internalclock signal. With this configuration, an operation like that in FIG. 35is made possible.

In the foregoing transmission systems, the description has been givenabout the data transmission between the buffer and the DRAM that aremounted on the memory module. However, the present invention is not atall limited thereto. For example, the present invention is alsoapplicable to a memory circuit other than a DRAM, e.g. a ROM. Further,the present invention can achieve high-speed data transmission even ifit is applied to the system that requires bidirectional datatransmission or that requires a strobe signal.

In the foregoing memory systems, the buffer and the plurality of DRAMsare mounted on each memory module, and reception/transmission of datasignals relative to the DRAMs on the memory module and transmission ofclocks and address/command signals relative to the DRAMs are all carriedout via the buffer on each memory module. Further, in the foregoing, thedescription has been mainly given about one-to-one datareception/transmission between the buffer and each of the DRAMs on eachmemory module.

However, for actually operating the foregoing memory module at highspeed, it is further necessary to process timing skews that aregenerated between data signals, and clocks and command/address signalsdepending on positions of the DRAMs on the memory module, and moreover,to perform matching of clock timings in the buffer relative to data thatare transmitted from the respective DRAMs and arrive at the buffer atdifferent timings.

Here, referring to FIG. 41, the foregoing point will be described morespecifically. A buffer 105 and a plurality of DRAMs 110 are mounted on ashown memory module 103. The package size of each DRAM 110 mounted onthe memory module 103 normally has a width of about 14 mm, and this sizeis considered to be maintained even if the generation is advanced. Whenthe DRAMs 110 each having such a size are mounted as shown in thefigure, for example, when the five DRAMs 110 are mounted at regularintervals of 9 mm, a wiring length of each of a clock line, acommand/address ling, and a DQ signal line between the far-end DRAM 110(denoted by 110F) and the buffer 105 is 65 mm, while a wiring lengththereof between the near-end DRAM 110 (denoted by 110N) and the buffer105 is 9 mm.

When the memory module 103 thus dimensioned is operated at a highfrequency of 800 MHz, a timing skew of a level that can not be ignoredrelative to an operation period (1250 ps) of the high-frequencyoperation (800 MHz) is generated at the far-end DRAM 110F due to adifference in signal propagation time between the clocks and thecommand/address signal, and the DQ signal.

More specifically, since the clocks and the command/address signal areinputted into the respective DRAMs 110 from the buffer 105 via thecommon wiring, an input capacitance of about 1.5 pF×2×5 is distributedon the wiring relative to the clocks and the command/address signal.Therefore, a signal unit propagation time (tPD) of the clocks and thecommand/address signal becomes about 14 ps/mm. On the other hands the DQsignal is transmitted/received between the buffer and the respectiveDRAMs 110 via one-to-one or one-to-two wiring, and therefore, an inputcapacitance of about 2.5 pF×2 is distributed on the wiring relative tothe DQ signal. Therefore, a signal unit propagation time tPD of the DQsignal becomes about 8 ps/mm, and thus it is understood that the signalunit propagation time of the DQ signal is shorter than the signal unitpropagation time of the clocks and the command/address signal.

Based on such a difference in signal propagation time between the clocksand the command/address signal, and the DQ signal, the timing skew ofthe level that can not be ignored relative to the operation period (1250ps) of the high-frequency operation (800 MHz) is generated at thefar-end DRAM 110F. In the shown memory system, a signal propagation timeof the clocks and the command/address signal upon writing is 910(=14×65) ps, while a signal propagation time of the DQ signal is 520(=8×65) ps. As a result, a timing skew of 390 ps is generated betweenthe clocks and the command/address signal, and the DQ signal at thefar-end DRAM 110F.

When a write command (WRT) is given to the far-end DRAM 110F in thestate where such a timing skew is generated, the write command isinputted into the DRAM at a phase of a buffer clock signal from thebuffer 105.

On the other hand, a data write operation in each DRAM 110 isimplemented synchronously with the buffer clock signal after thereception of the write command. This means that data received at leadingedges of the data reception DRAM internal clocks should be matched withthe phase timing of the buffer clock signal during one cycle.

For example, data received at leading edges of the data reception DRAMinternal clocks are matched with the clock signal phase timing attrailing edges of the buffer clock signal, while data received attrailing edges are matched with the clock signal phase timing at leadingedges of the buffer clock signal. As a result, internal data arealternately produced. When shifting matching of such data from onetiming to another timing, a setup time and a hold time are required.

In the system shown in FIG. 41, a setup time and a hold time at thenear-end DRAM 110N for shifting matching of data received at the timingof the data reception DRAM internal clocks to the buffer clock timingare 679 ps and 571 ps, respectively, while a setup time and a hold timeat the far-end DRAM 110F are 1015 ps and 235 ps, respectively.

As clear from this, since a timing skew between the clock signal and theDQ signal is 54 ps, i.e. small at the near-end DRAM, uniform margins areobtained for the setup time and the hold time, while, the hold timebecomes 235 ps (0.19 clock period), i.e. short, at the far-end DRAM 110Fdue to the skew of 390 ps, so that a sufficient time margin can not beobtained.

Further, DQ signals transmitted from the respective DRAMs in response toa read (READ or RED) command arrive at the buffer 105 at differentarrival times due to a difference between a propagation time of theclock signal (equal to a propagation time of the command) and apropagation time of the DQ signal. For example, a propagation time ofthe clock signal (command) to the near-end DRAM 110N is 126 ps and apropagation time of the DQ signal to the buffer 105 from the near-endDRAM 110N is 72 ps, while a propagation time of the clock signal(command) to the far-end DRAM 110F is 910 ps and a propagation time ofthe DQ signal to the buffer 105 from the far-end DRAM 110F is 520 ps.

Assuming that a latency from a read command to data output is equalamong the respective DRAMs, e.g. eight clocks here, the total signaltwo-way propagation time at the near-end DRAM 110N is 198 ps, while thetotal signal two-way propagation time at the far-end DRAM 110F is 1430ps, i.e. a difference therebetween is 1230 ps.

Therefore, at the buffer 105, it is necessary to match the data ofdifferent arrival times with the timing of the clock signal again, andtransfer them to the memory controller. Further, as clear from theforegoing, data from the near-end DRAM 110N and data from the far-endDRAM 110F arrive spanning different clock cycles within the buffer 105.Therefore, it is necessary to judge at the buffer 105 per data from eachDRAM 110 as to which cycle it should be matched with.

Hereinbelow, referring to the drawings, description will be given aboutexamples of the present invention that take the foregoing skew intoaccount.

In the following examples, it is assumed that a clock signal fed to eachDRAM (herein, called “buffer clock signal”) is produced by dividingclocks fed to the buffer 105 (herein, called “global clocks”) to half infrequency for the purpose of processing the foregoing skew, and a DPSsignal is transmitted at a frequency equal to that of the producedbuffer clock signal. Therefore, the command/address signal istransmitted/received synchronously with leading and trailing edges ofthe clock signal. Further, a data signal is received/transmittedsynchronously with the DPS signal at a transfer rate that is four timesa frequency of the clock signal.

Referring to FIG. 42, there is shown a configuration of a DRAM that isused in a memory system according to a first example of the presentinvention, wherein write/read data phase signals (WDPS/RDPS) areinputted/outputted via mutually different wirings.

The DRAM 110 shown in FIG. 42 is provided with a command/addressreception clock generating circuit (DLL) 500 and a domain crossingcircuit 501, which differs from the DRAMs 110 shown in other figures.The shown clock generating circuit (DLL) 500 and domain crossing circuit501 operate in response to reception of a buffer clock signal and acommand/address signal each having a frequency of 400 MHz from thebuffer, respectively.

In the shown example, the command/address signal is received into theDRAM 110 at the timing of the buffer clock signal (hereinafter, it mayalso be referred to simply as “clock signal”), and delivered to dataphase clocks within the DRAM 110 produced based on the data phase signal(WDPS). Through this, the command/address signal becomes an internalcommand produced based on the data phase (WDPS), and thereafter, aninternal read/write operation of the DRAM 110 is carried out accordingto this internal command. This means that the internal read/writeoperation of the DRAM 110 is implemented synchronously with the dataphase of the WDPS.

Here, for allowing the phase of the clocks in the DRAM 110 to allocatemargins to a setup time and a hold time relative to the phase of thedelivery-destination WDPS, the WDPS signal is delayed by one clock(represented by 1tCK) of the global clocks, i.e. by 180 degrees of thedivider clocks, in the buffer 105.

Referring to FIG. 43, there is shown a concrete configuration of thedomain crossing circuit 501 provided in the DRAM 110. The shown domaincrossing circuit 501 is a circuit for domain-crossing thecommand/address signal from the phase of the buffer clock signal to theWDPS phase, and comprises a first latch circuit 511 and a second latchcircuit 512. Specifically, the first latch circuit 511 comprises tworeceivers for receiving a command signal according to 0-degree phaseclocks and 180-degree phase clocks and latching it, while the secondlatch circuit 512 comprises two flip-flop circuits for holding thecommand signal from the first latch circuit 511 according to 0-degreedata phase clocks and 180-degree data phase clocks.

Here, the 0-degree and 180-degree phase clocks are produced at thecommand/address reception clock generating circuit 500 shown in FIG. 42,and represent 0-degree and 180-degree phases of the received bufferclock signals, respectively. On the other hand, the 0-degree and180-degree data phase clocks represent 0-degree and 180-degree phases ofthe write data phase signal (WDPS), respectively.

As shown in FIG. 42, the 0-degree and 180-degree data phase clocks areproduced at a clock reproduction and phase adjusting circuit (DLL) 205that operates in response to the WDPS.

As clear from this, it is understood that the shown domain crossingcircuit 501 shifts synchronization of the command signal (or the addresssignal) with the 0-degree or 180-degree phase of the buffer clock signalto synchronization thereof with the 0-degree or 180-degree phase of thedata phase signal (WDPS), and outputs it as a DRAM internalcommand/address signal.

Referring to FIG. 44, there is shown a concrete configuration of thebuffer 105 forming the first example of the present inventioncooperatively with the DRAM 110 shown in FIG. 42, wherein the buffer 105implements transmission/reception of the data signal DQ relative to theDRAM 110 of FIG. 42. The shown buffer 105 has a clock dividing/phasecomparing adjusting circuit 601 that operates in response to receptionof global clocks given from a memory controller (not shown). The clockdividing/phase comparing adjusting circuit 601 outputs buffer clocksobtained by dividing the global clocks to half in frequency, to the DRAM110 as a clock signal, while outputs WDPS for DRAMs. In the figure,there is shown only a portion of outputting the WDPS for the far-endDRAM 110F.

Further, the shown clock dividing/phase comparing adjusting circuit 601internally outputs data output buffer internal clocks and WDPS bufferinternal phase clocks to a DQ output driver 301 and a domain crossingcircuit 602, respectively. Here, the WDPS buffer internal phase clocksrepresent 0-degree, 90-degree, 180-degree and 270-degree phases of theWDPS for the far-end DRAM 110F.

On the other hand, a clock reproducing/phase adjusting circuit 305,which operates in response to reception of an RDPS being a data phasesignal from the far-end DRAM 110F, produces data reception bufferinternal phase clocks representing 0-degree, 90-degree, 180-degree and270-degree phases of the RDPS, and feeds them to the domain crossingcircuit 602.

The domain crossing circuit 602 in the buffer 105 comprises afirst-stage data latch circuit 611 and a second-stage data latch circuit612. Specifically, the domain crossing circuit 602 is a circuit fordomain-crossing from the RDPS phase to the WDPS phase and, as shown inFIG. 45, comprises the first-stage data latch circuit 611 for receivinga data signal DQ read from the DRAM 110 according to buffer internalphase clocks produced synchronously with 0-degree, 90-degree, 180-degreeand 270-degree phases of the RDPS, and latching it, and the second-stagedata latch circuit 612 for latching an output of the first-stage datalatch circuit 611. The second-stage data latch circuit 612 comprisesflip-flop circuits respectively latching according to WDPS bufferinternal phase clocks (270, 0, 90 and 180 degrees) produced at the clockdividing/phase comparing adjusting circuit 601 shown in FIG. 44, andlatches the output from the first-stage data latch circuit 611 at thephases of the WDPS buffer internal phase clocks, then outputs it as abuffer internal data signal.

Referring to FIG. 46, description will be given about an operation ofthe shown example upon writing. Here, description will be given about anoperation between the buffer 105 and the near-end DRAM 110N uponwriting. Herein, it is assumed that, for matching the command/addresssignal with the global clocks in each DRAM 110, i.e. for shifting thecommand/address signal from the phase domain of the buffer clocks to thephase domain of the WDPS, the buffer 105 outputs the WDPS to thenear-end DRAM 110N by delaying the WDPS by one system clock time phase(1250 ps), and a write latency (WL) is six system clocks.

As shown in the figure, when global clocks of 800 MHz (see the firstline) are received, the clock dividing/phase comparing adjusting circuit601 of the buffer 105 outputs buffer clocks of 400 MHz (see the secondline). Synchronously with the buffer clocks, a write command (WRT) isoutputted to the near-end DRAM 110N. On the other hand, a write phasesignal (WDPS) of 400 MHz is outputted to the near-end DRAM 110N with adelay of a phase corresponding to one global clock (1250 ps), i.e. witha delay of ½ phase of the buffer clock signal. After the foregoing WL, awrite data signal (DQ) is outputted to the near-end DRAM 110Nsynchronously with the WDPS.

On the other hand, at the near-end DRAM 110N, as described before, thebuffer clocks and the write command (WRT) arrive in a propagation timeafter 126 ps, while the WDPS arrives in a 54 ps-shorter propagationtime.

As shown in FIG. 42, at the near-end DRAM 110N, the command/addressreception clock generating circuit 500 generates 0-degree and 180-degreephase clocks representing 0 degrees and 180 degrees of the receivedbuffer clocks. Further, the clock reproducing/phase adjusting circuit205 of the near-end DRAM 110N receiving the WDPS generates 0-degree and180-degree phase data phase clocks representing 0-degree and 180-degreephases of the WDPS.

In the shown example, the command/address signal received at the DRAMsynchronously with the clock signal is subjected to domain crossing from0-degree phase clocks (phase of buffer clocks) to 0-degree phase dataphase clocks (0-degree phase of WDPS) and, as a result, an internalwrite command (WRT) is produced synchronously with the 0-degree phasedata phase clocks. This means that the domain crossing from the bufferclock phase to the WDPS phase has been implemented, and writing of thedata signal (DQ) is carried out after 6 WL in response to the internallyproduced write command (WRT).

A setup time and a hold time of the thus configured near-end DRAM 110Nfor shifting the command/address signal from the clock phase to the dataphase are 1196 ps and 1304 ps, respectively, and therefore, it is seenthat a sufficient time margin can be ensured.

The near-end DRAM 110N produces an RDPS in phase with the received WDPSand outputs it to the buffer 105, which arrives at the buffer 105 aftera propagation time of 144 ps.

Referring to FIG. 47, there is shown an operation, upon writing, betweenthe buffer 105 and the far-end DRAM 110F in the memory system accordingto the foregoing example. As shown in the figure, a write command (WRT)is outputted synchronously with buffer clocks of 400 MHz, while a WDPSis outputted with a delay of ½ phase of a 1250 ps delayed buffer clocksignal relative to the buffer clocks. The write command (WRT) and thebuffer clocks, and the WDPS reach the far-end DRAM 110F after a lapse ofdifferent delay times. They are received at the far-end DRAM 110F in thestate where the foregoing skew of 390 ps is generated between the bufferclocks and the WDPS. At the far-end DRAM 110F, the received writecommand WRT is caused to match with the timing of the received WDPS tothereby produce a DRAM internal command (WRT) synchronously with thereceived WDPS, and a data signal (DQ) is written after 6 WL from theDRAM internal command.

As shown in the figure, a hold time and a setup time of the thusconfigured far-end DRAM 110F for shifting the command/address signalfrom the clock phase to the data phase can be 1640 ps and 860 ps,respectively. Accordingly, it is seen that a sufficient timing margincan be ensured.

Further, as shown in the figure, the far-end DRAM 110F having receivedthe WDPS outputs an RDPS to the buffer 105 synchronously with the WDPS,wherein the RDPS has the same phase as the WDPS. After a lapse of 1040ps subsequently to the production of the WDPS, the buffer 105 receivesthe RDPS having the corresponding phase from the far-end DRAM 110F. Inthis example, the RDPS has the same phase as the WDPS. Accordingly,0-degree phase of the RDPS corresponds to 0-degree phase of the WDPS,90-degree phase of the RDPS corresponds to 90-degree phase of the WDPS,and likewise, 180-degree and 270-degree phases of the RDPS correspond to180-degree and 270-degree phases of the WDPS, respectively.

Now, referring to FIG. 48, description will be given about a readoperation in the memory system according to the foregoing example,wherein the buffer 105 outputs a read command (RED) to the far-end DRAM110F synchronously with the buffer clocks. As described above, when atime of 1040 ps has elapsed after the transmission of the WDPS, the RDPShaving the corresponding phase arrives at the buffer 105 from thefar-end DRAM 110F.

On the other hand, on the side of the far-end DRAM 110F, synchronouslywith the received WDPS, the RDPS having the same phase is outputted tothe buffer 105. The buffer 105 outputs the read command (RED) to thefar-end DRAM 110F synchronously with buffer clocks. The far-end DRAM110F receives the read command at the timing of the buffer clock signaland delivers it to the data phase clocks produced based on the WDPS. Asa result, the read command signal becomes an internal command producedbased on the data phase (WDPS), and thereafter, an internal readoperation of the DRAM 110F is implemented by this internal read command.After a lapse of eight global clocks from the received RED, a datasignal (DQ) is read out. The read-out data signal is outputted to thebuffer 105 from the far-end DRAM 110F synchronously with the RDPS and,after 520 ps, received at the buffer 105.

In this configuration, the timing margin for domain crossing from theRDPS phase to the WDPS phase in the buffer 105 is 835 ps, and therefore,it is understood that the sufficient timing margin can be obtained.

Further, referring to FIG. 49 and FIG. 44, description will be givenabout an operation, upon reading, in the buffer 105 in the foregoingexample. Here, it is assumed that a data signal (DQ) is read from thefar-end DRAM 110F. At the buffer 105, the read data signal (DQ) isreceived synchronously with the received RDPS. The buffer 105 shown inFIG. 44 produces, from the RDPS, four-phase data reception bufferinternal clocks (0, 90, 180 and 270 degrees) representing phases of theRDPS, and feeds them to the first-stage data latch circuit 611 of thedomain crossing circuit 602. Therefore, the data signal (DQ) from thefar-end DRAM 110F is stored into the first-stage data latch circuit 611synchronously with those four-phase data reception buffer internalclocks, then fed to the second-stage data latch circuit 612.

Four-phase buffer internal phase clocks obtained from the WDPS (globalclocks) produced at the buffer 105 are given to the second-stage datalatch circuit 612 from the clock dividing/phase comparing adjustingcircuit 601, and an output of the first-stage data buffer 611 is storedinto the second-stage data latch circuit 612 according to the four-phasebuffer internal phase clocks. As a result, the data signal (DQ) readfrom the far-end DRAM 110F is caused to match with the internal clocksproduced in the buffer 105, so as to be outputted to the memorycontroller from the buffer 105.

Now, referring to FIG. 50, description will be given about an operationof the buffer 105 when processing data signals (DQ) from the near-endand far-end DRAMs 110N and 110F upon reading. It is assumed that readcommands (RED) and WDPS delayed by ½ phase relative to buffer clocks areoutputted to the near-end and far-end DRAMs 110N and 110F from thebuffer 105 synchronously with the buffer clocks. In this case, as shownin the figure, an RDPS signal having the same phase as a correspondingphase of a WDPS signal is inputted into the buffer 105 at timing delayedby 144 ps from the near-end DRAM 110N, while it is inputted into thebuffer 105 at timing delayed by 1040 ps from the far-end DRAM 110F.Here, assuming that the buffer 105 is set to start a data take-inoperation at a time instant when (8+2.5) global clock time elapses afterproduction of the read command (RED), hold times for shifting the timingof the data signals (DQ) that are read out synchronously with the RDPSof the near-end and far-end DRAMs, from the RDPS phase to the WDPSphase, i.e. the clock phase, in the buffer 105 are 770 ps and 1665 ps,respectively, and setup times therefor are 1731 ps and 835 ps,respectively, and therefore, it is understood that sufficient timemargins are ensured.

The foregoing operation will be described in a more generalized manner.A buffer clock signal obtained by n-dividing (dividing by n) a systemclock (global clock) signal in frequency, and a data phase signal (WDPS)having a frequency equal to that of the buffer clock signal are fed tothe DRAMs from the buffer 105. On the other hand, command/addresssignals are transmitted from the buffer 105 while being matched with thebuffer clock signal. When the command/address signals transferred in aperiod are m times at maximum, each command/address signal is receivedby one of internal clock signals produced per 1/m phase from the timingof the buffer clock signal at the DRAM.

On the other hand, in each DRAM 110, the command/address signal isdelivered to previously associated one of internal data phase clocksthat are internally produced per 1/m phase, likewise, from the timing ofthe data phase signal (WDPS) transmitted from the buffer 105, so that aninternal command/address signal is produced.

Data signals written into the respective DRAMs 110 are transmitted tothe DRAMs 110 from the buffer 105 while being matched with the timing ofthe data phase signal (WDPS). When the data signals transferred in aperiod are k times at maximum, the data signal is received at each DRAM110 and stored therein by one of internal clock signals that areproduced at the DRAM 110 per 1/k phase from the timing of the data phasesignal (WDPS) transmitted from the buffer 105.

On the other hand, the data signal read from each DRAM 110 istransmitted from the DRAM 110 while being matched with the timing of thedata phase signal (RDPS), and received at the buffer 105 by one ofinternal clock signals produced per 1/k phase from the timing of thedata phase signal (RDPS) transmitted from the DRAM 110. This RDPS isdelivered to previously associated one of internal clocks produced per1/k phase from the timing of the data phase signal (WDPS) that isoriginally produced in the buffer 105, so that an internal read datasignal is produced.

In this case, the command/address signal is transmitted to the buffer105 synchronously with leading and trailing edges of the buffer clocksignal, and taken into the DRAM synchronously with leading and trailingedges of the buffer clock signal.

Referring to FIG. 51, there is shown a DRAM 110 that is used in a memorysystem according to a second example of the present invention. The DRAM110 according to this example is configured to take in a data signalusing phase clocks produced from a WDPS and deliver it to phase clocksproduced from a buffer clock signal. Therefore, the shown DRAM 110comprises a clock reproducing/phase adjusting circuit 521 that operatesin response to reception of the WDPS, and the clock reproducing/phaseadjusting circuit 521 is connected to a reception replica 523 and areception phase comparing circuit 525. Under the control of a receptionphase adjusting signal from the reception phase comparing circuit 525,the shown clock reproducing/phase adjusting circuit 521 producesfour-phase data reception DRAM internal phase clocks (0, 90, 180 and 270degrees) from the WDPS and feeds them to a first-stage data latchcircuit 527 of a domain crossing circuit 501.

On the other hand, the buffer clock signal is given to a clockreproducing/phase adjusting circuit (DLL) 205 which produces four-phasephase clocks therefrom and feeds them to a second-stage data latchcircuit 529 of the domain crossing circuit 501.

Referring also FIG. 52, the first-stage data latch circuit 527 of thedomain crossing circuit 501 is given a data signal (DQ) from the buffer105, and further given from the clock reproducing/phase adjustingcircuit 521 four-phase data reception DRAM internal phase clocksproduced from the WDPS. Therefore, the first-stage data latch circuit527 composed of four receivers/latches receives the data signal (DQ) attiming of the four-phase data reception DRAM internal clocks and latchesit, and feeds outputs thereof to the second-stage data latch circuit 529composed of four flip-flop circuits, respectively.

Four-phase DRAM internal phase clocks are respectively given to the fourflip-flop circuits of the second-stage data latch circuit 529, and theoutputs from the first-stage data latch circuit 527 are stored accordingto the four-phase DRAM internal phase clocks and outputted as a DRAMinternal data signal.

Further, the clock reproducing/phase adjusting circuit 205 producestwo-phase phase clocks of 0 and 180 degrees from the buffer clock signaland feeds them to a command/address receiver 531. The command/addressreceiver 531 takes in a command/address signal according to thetwo-phase phase clocks and outputs it as an internal command/addresssignal. Accordingly, the internal command/address signal is produced atthe buffer clock phase, and an internal read/write operation of the DRAMis implemented synchronously with the buffer clock phase.

Referring to FIG. 53, there is shown a specific example of the buffer105 that is used while being connected to the foregoing DRAM 110. Aclock dividing/phase comparing adjusting circuit 601 included in theshown buffer 105 feeds buffer internal four-phase phase clocks to adomain crossing circuit 602, and further outputs data output bufferinternal four-phase clocks to a DQ output driver 301, which differs fromthe buffer 105 shown in FIG. 44. Further, the shown domain crossingcircuit 602 is given data reception buffer internal four-phase clocksproduced based on the RDPS from a clock reproducing/phase adjustingcircuit 305.

Referring also to FIG. 54, a first-stage data latch circuit 611 of thedomain crossing circuit 602 shown in FIG. 53 comprises four receiversfor receiving a data signal (DQ) according to the four-phase datareception buffer internal phase clocks and latching it, and outputs ofthe respective receivers are fed to four flip-flop circuits forming asecond-stage data latch circuit 612, respectively. These flip-flopcircuits latch the outputs of the first-stage data latch circuitaccording to the four-phase buffer internal phase clocks. As shown inthe figure, the outputs received and latched in the first-stage datalatch circuit 611 according to the 0, 90, 180 and 270-degree datareception buffer phase clocks, i.e. the clocks representing the RDPS,are latched in the second-stage data latch circuit 612 according to the270, 0, 90 and 180-degree internal phase clocks, respectively, andtherefore, it is understood that the data signal is latched by thedifferent phase clocks. In other words, in the shown example, it is seenthat shifting to a 90-degree advanced phase is performed in the phase ofthe buffer clock signal.

Referring to FIG. 55, description will be given about a write operationbetween the buffer 105 and the near-end DRAM 110N. The buffer 105outputs a WDPS to the near-end DRAM 110N. For ensuring a time margin forshifting a data signal (DQ) from WDPS phase domain to clock phase domainin the DRAM 110, the WDPS has a phase that is advanced by 90 degrees (½clock in terms of global clocks; 625 ps) relative to the buffer clocksignal.

In the figure, a write command (WRT) is outputted to the near-end DRAM110N from the buffer 105 synchronously with the buffer clocks. On theother hand, after a write latency corresponding to six clocks of theglobal clocks, the data signal (DQ) is outputted from the buffer 105synchronously with the WDPS.

The buffer 105 outputs the buffer clocks and the write command (WRT)synchronous with the buffer clocks, and further outputs the WDPS whilematching it with the buffer clocks.

In this event, the write command (WRT) and the WDPS (i.e. DQ) arereceived at the near-end DRAM 110N while having a propagation delaydifference of 54 ps therebetween.

After 6 WL (write latency) from the received write command, when thedata signal (DQ) is outputted from the buffer 105 synchronously with theWDPS, it is inputted into the DRAM 110N according to the data phaseclocks produced from the WDPS, and delivered to the phase clocksproduced from the buffer clock signal. Herein, a hold time and a setuptime for domain crossing from the data phase to the clock phase are 1821ps and 679 ps, respectively. The shown near-end DRAM 110N outputs theRDPS to the buffer 105 at the timing of the received buffer clocks.After 72 ps, i.e. after 198 ps from a corresponding phase of the globalclocks, the RDPS is inputted into the buffer 105.

Referring to FIG. 56, there is shown a write operation relative to thefar-end DRAM 110F. In this case, assuming that there exists a skewpropagation delay time difference of 390 ps between a write command(WRT) and a data signal (DQ) which are received at the far-end DRAM110F, there also exists a like skew between buffer clocks and a WDPS.Taking this into account, the phase of the WDPS is advanced by 90degrees, and domain crossing from the WDPS phase to the buffer clockphase is carried out. As a result even in the far-end DRAM 110F, asshown in the figure, a hold time of 1485 ps and a setup time of 1015 psare ensured for domain crossing from the data phase to the clock phase,so that a sufficient timing margin is obtained.

Further, upon reading, as shown in FIG. 57, the DRAM 110 transmits anRDPS to the buffer 105 so as to be in phase with the buffer clock phase,and a data signal (DQ) is transmitted to the buffer 105 while beingmatched with the RDPS. The buffer 105 takes in the data signal accordingto a phase clock signal produced from the RDPS. In this manner, bydelivering the data signal to the phase clock signal produced based onthe clock signal within the buffer 105, it is possible to match it withthe clock phase in the buffer 105.

In the buffer 105, for allowing the phase of the RDPS in the buffer 105to allocate margins to a setup time and a hold time relative to thephase of the delivery-destination clocks, the delivery is carried outsuch that 0 degrees to the RDPS correspond to 270 degrees of the clocksignal.

Through this operation, as shown in FIG. 58, when the read data from thenear-end and far-end DRAMs are received at the buffer 105, a sufficientsetup time and hold time can be ensured. In the shown example, a holdtime of 823 ps and a setup time of 1677 ps can be ensured in thenear-end DRAM 110N, while a hold time of 2055 ps and a setup time of 445ps can be ensured in the far-end DRAM 110F. In the shown example, in thedata signal reading operation, the total latency is equal to the sum ofa read-out time and 1.5 clocks in the DRAM.

As clear from the foregoing, the command/address reception clockgenerating circuits 500 and 521, the domain crossing circuits 501, andthe clock reproducing/phase adjusting circuits 205 in the DRAMs 110shown in FIGS. 42 and 51 operate as DRAM side circuits for absorbing askew between the data signal and the command/address signal, while theclock dividing/phase comparing adjusting circuits 601, the domaincrossing circuits 602, and the clock reproducing/phase adjustingcircuits 305 in the buffer 105 shown in FIGS. 44 and 52 operate asbuffer side circuits for absorbing the skew.

In the foregoing two examples, the clock signal and the data phasesignal (W/RDPS) fed to each DRAM are produced in the buffer 105 by2-dividing (dividing to half or dividing by 2) the system clock signal(i.e. global clocks) in frequency. Further, in each DRAM and the buffer105, the clock phase signal and the data phase signal are produced per ½phase in case of the command/address signal and per ¼ phase in case ofthe data signal. Further, the clock phase signal and the data phasesignal internally produced and having different phases are associatedwith each other to thereby shift the timing of the received signalbetween the clocks. In this case, since the period of each of theassociated signal is twice that of the system clock signal, margins tothe setup time and the hold time can be ensured relative to thedelivery-destination phase signal as described above.

In this case, the margins to the setup time and the hold time areideally such that edges of the phase signal taking in the signal to bedelivered are located just at the middle positions between edges of thedelivery-destination phase signal. However, in case of signaltransmission from the buffer to the DRAM, an adjustment may be performedto retard or advance the phase of the WDPS in the buffer relative to theclock signal, thereby to more approximate it.

Further, when matching the DQ signals from the DRAMs in the buffer, adelivering-side phase signal may be selected such that edges of the RDPSfrom the far-end and near-end DRAMs approximate the middle positions ofthe WDPS or the clock signal serving as the delivery-destination phasesignal. In the foregoing examples, it is clear that the 270-degree phasesignal of the WDPS or the clock signal is set to correspond to the0-degree phase signal of the RDPS, thereby to achieve matching of thetiming of the DQ signals from the DRAMs.

Further, a flight time that is not synchronous with clocks on the moduleuntil the DQ signal is transferred to the buffer from the DRAM becomes atime for the data signal to go and return between the buffer and theDRAM in case of the first example, while it becomes the sum of a timefor the read command to be transmitted from the buffer to the DRAM and atime for the data signal to be transmitted from the DRAM to the bufferin case of the second example. It becomes 1040 ps at maximum (in case ofthe far-end DRAM) in the first example, while it becomes 1430 ps atmaximum in the second example. By 2-dividing the system clock signal infrequency, it becomes possible to perform the processing (matching withthe original clock phase on the buffer) in one cycle (2500 ps).

Referring to FIG. 59, description will be given about a memory systemaccording to a third example of the present invention. In this example,a DPS (Data Phase Signal) is used and, while suppressing the increase ofthe number of wirings, transmission/reception of a DPS of a differentialsignal is made possible. This example differs from the other examples inthat an RDPS transmitted from each DRAM and a WDPS transmitted from abuffer 105 are transmitted/received via a common signal line, and acontrol signal (indicate) is transmitted to the DRAMs 110 from thebuffer 105. This control signal (indicate) is a signal for switching, onthe side of the DRAM 110, between a time period for receiving a dataphase signal (WDPS) from the buffer 105 and a time period fortransmitting a data phase signal (RDPS) to the buffer 105. On the otherhand, the buffer 105 switches reception/transmission of a data phasesignal (DPS) in the buffer 105 according to a control signal (indicate)of itself.

As shown in FIG. 59, since the control signal can be shared among theDRAMs 110 on the memory module, wiring for the control signal (indicate)is increased only by one.

In the memory system (i.e. memory module 103) according to the foregoingthird preferred embodiment, it is necessary to configure the drivercircuit in open-drain mode when the signal line is shared with the RDPSand WDPS. However, in this example, it may also be a CMOS push-pulldriver, or a differential signal may be used, so that the timingaccuracy can be improved.

Referring to FIG. 60, there is shown a configuration of the DRAM 110that is used in this example, while FIG. 61 shows a configuration of thebuffer 105 likewise used in this example. As clear from FIG. 61, thebuffer 105 is provided with a DPS control signal producing circuit 701,and a control signal (indicate) is transmitted to the DRAMs 110 from theDPS control signal producing circuit 701, while an internal controlsignal is outputted to a clock dividing/phase comparing adjustingcircuit 601, a clock reproducing/phase adjusting circuit 305, and areception phase comparing circuit 306 from the DPS control signalproducing circuit 701.

On the other hand, the DRAM 110 shown in FIG. 60 is provided with a DPScontrol circuit 541 that, in response to reception of the control signal(indicate), switches a mode of a DPS driver 207, and changes the stateof a clock reproducing/phase adjusting circuit 521 and a reception phasecomparing circuit 525. Inasmuch as the other components have alreadybeen explained, no details thereof are given here.

Referring to FIG. 62, there is shown timing for switching between a timeperiod for transmitting the data phase signal from the buffer 105according to the control signal (indicate) transmitted from the buffer105, and a time period for transmitting the data phase signal from theDRAM according to the control signal. In the shown example, both timeperiods are switched alternately.

FIG. 63 shows a case wherein a switching time period of the indicate isset long during initialization for allowing the DLLs to lock on, whilethe switching time period is set shorter during a normal operation forfine adjustment as compared with that during the initialization. In thismanner, the buffer 105 can set the switching time period to be longduring the initialization to thereby allow the DLLs to lock on, while itcan set the switching time period to be shorter during the normaloperation as compared with that during the initialization to therebydeal with fluctuation caused by operation noise. In this configuration,although a phase locking time at the DRAM becomes long during theinitialization that requires fine adjustment, inasmuch as variation inphase due to operation noise is small, no problem is raised.

In the foregoing examples, the setup time and the hold time areestimated assuming that the period, i.e. the effective operationfrequency, of the global clocks is 800 MHz. If the frequency is relaxed,the setup time and the hold time are also relaxed correspondingly, andtherefore, the foregoing phase adjustment may be performed with themaximum frequency expected upon designing the memory module.

In the foregoing examples, the description has been given only about thememory systems in which the buffer is provided on the memory module. Inother words, the description has been given only about the memorysystems that can increase the number of memory modules. However, thepresent invention is also applicable to a memory system having aconfiguration in which a single memory module mounted thereon with nobuffer is controlled by a memory controller. In the memory system ofthis type, the functions of the buffers in the foregoing examples may beimplemented by the memory controller.

Referring to FIG. 64, there is shown one example of the foregoing memorysystem as still another example of the present invention. The shownmemory system 1000 comprises a memory controller 1011, a clock generator102, and a single module 1031 on which four DRAMs 110 (1 to 4) and fiveDRAMs 110 (1′ to 5′) are mounted on the left side and the right side,respectively. In other words, the shown memory system 1000 issubstantially the same as each of the memory systems shown in otherfigures wherein the memory controller 1011 is provided instead of thebuffer 105. In the shown example, the memory controller 1011 and theDRAMs 110 are respectively connected to each other via data wirings DQhaving the same length, and arrival times of data signals DQ from thememory controller 1011 at the respective DRAMs 110 are substantially thesame.

On the module 1031, the left-side four DRAMs 110 (1 to 4) are connectedto the memory controller 1011 via common clock wiring and commoncommand/address wiring, while the right-side five DRAMs 110 (1′ to 5′)are also connected to the memory controller 1011 via other common clockwiring and common command/address wiring. That is, it is seen that theleft-side DRAMs 110 (1 to 4) and the right-side DRAMs 110 (1′ to 5′) areconnected to the memory controller 1011 via the separate clock wiringsand command/address wirings.

With respect to the DRAMs 110 (4) and (5′) disposed at far ends in thememory system having the shown topologies, there are large differencesin wiring length between the clock wiring and the address/command wiringrelative to the memory controller 1011, and the data wiring DQ relativeto the memory controller 1011.

Therefore, a propagation delay difference between the clock signal(command/address signal) and the data signal DQ from the memorycontroller 1011 at the DRAMs 110 (4) and (5′) becomes larger than thatin the foregoing modules.

For example, in the shown example, assuming that a DRAM pitch is 13 mmand a signal unit propagation time tPD is 14 ps/mm, a delay of thecommand/address signal on the module 1031 becomes 728 ps (13×4×14) atthe DRAM 110 (4), while it becomes 910 ps (13×5×14) at the DRAM 110(5′). Assuming that propagation delays of the clock and command/addresssignals and the data signal DQ from the memory controller 1011 to inputterminals of the module 1031 are equal to each other, the foregoingdelays on the module 1031 become skew differences between thecommand/address signal and the data signal DQ, respectively.

The memory system 1000 according to the fourth example of the presentinvention processes those skew differences using the domain crossingtechnique that employs the foregoing DPS (Data Phase Signal). Referringto FIG. 65, there is shown a write operation in the memory system 1000shown in FIG. 64. First, the clock generator 102 generates referenceclocks (i.e. system clocks) of 800 MHz and feeds them to the memorycontroller 1011. The memory controller 1011 divides the reference clocks(system clocks) to half in frequency to produce system clocks of 400MHz, while produces a write command (WRT) synchronously with theproduced system clocks.

Further, in the memory controller 1011 shown in FIG. 64, a DPS (WDPS)advanced by 90 degrees relative to the clock signal is produced, andthis WDPS is transmitted to the DRAMs 110. In FIG. 65, there is shown acase wherein the WDPS is transmitted only to the DRAMs 110 (1 to 5′). Byproducing the DPS having an advanced phase relative to the clock signal,there can be ensured margins to a setup time and a hold time fordomain-crossing a command/address signal from the clock phase to the DPSphase, i.e. the data signal DQ phase in the DRAM 110. That is, by usingthe DPS with the shifted phase relative to the clock signal, it ispossible to perform timing adjustment for the domain crossing.

In FIG. 65, when the write command (WRT) is received at the DRAM 110(1′) synchronously with the clock signal, the WRT is caused to matchwith the DPS received at the DRAM 110 (1′) so as to be produced as aDRAM internal command signal (DRAM internal Command). After a lapse of 6write latency time subsequently to the production of the DRAM internalcommand signal, a data signal write operation is implemented in the DRAM110 (1′).

On the other hand, the clock signal and the WRT are given to the DRAM110 (5′) with a delay as compared with the DRAM 110 (1′), and the DPS isalso given thereto with a delay of 965 ps relative to the clock signal.In this state, at the DRAM 110 (5′), the WRT is caused to match with theDPS so as to be produced as an internal command signal (DRAM internalCommand). As clear from FIG. 65, it is understood that, by implementingthe foregoing domain crossing, a sufficient setup time and hold time areensured in the DRAMs 110 (1′) and (5′).

Referring to FIG. 66, there is shown a read operation in the memorysystem 1000 shown in FIG. 64. Like in the write operation, the memorycontroller (MC) 1011 produces a read command (RED) synchronously withthe clock signal of 400 MHz. Further, the memory controller (MC) 1101produces a DPS (RDPS) having a phase advanced by 90 degrees relative tothe clock signal.

The clock signal (CLK) and the read command (RED) from the memorycontroller 1011 arrive at the DRAMs 110 (1′ to 5′) after mutuallydifferent propagation delay times, while the DPS arrives at the DRAMs110 (1′ to 5′) at substantially the same timing via the equal-lengthdata wirings.

Taking the far-end DRAM 110 (5′) as an example, the DRAM 110 (5′)receives the read command (RED) synchronously with the clock signal, andfurther receives the DPS. Like the DPS given to the other DRAMs 110, thesubject DPS is fed to the far-end DRAM 110 (5′) after a lapse of a delaytime of 700 ps subsequently to the production thereof at the memorycontroller (MC). In the far-end DRAM 110 (5′), the RED receivedsynchronously with the clock signal is caused to match with the DPSreceived at the far-end DRAM 110 (5′), so as to be produced as aninternal command signal (DRAM internal Command). In this manner, thedomain crossing is carried out from the timing of the clock signal tothe timing of the DPS.

On the other hand, in the memory system 1000 shown in FIG. 64, arrivaltimes of data signals DQ from the memory controller 1011 at therespective DRAMs 110 are substantially the same. However, in the memorycontroller 1011, it is necessary to identify a data signal DQ receivedfrom each DRAM 110 as to which of the read commands (RED) the receiveddata DQ corresponds to. Accordingly, the memory controller 1011 receivesthe DPS from each DRAM 110 and causes the timing of the received DPS tomatch with the timing of a WDPS of the memory controller (MC), i.e.performs the domain crossing. At the memory controller (MC) 1011, a datasignal DQ read from the DRAM 110 is received synchronously with a DPS(R)from the DRAM 110, and caused to match with the timing of the DPS(W) ofthe memory controller (MC) 1011. That is, at the memory controller (MC)1011, the data signal DQ received at the phase of the DPS(R) is shiftedto the phase of the DPS(W), i.e. returned to the phase of the clocksignal.

Therefore, in the memory controller (MC) 1011, by counting the number ofclocks from the issuance of the read command (RED), it is possible toidentify the data signal DQ as to which of the read commands (RED) itcorresponds to.

In FIG. 66, it is assumed that an interval between the memory controller(MC) 1011 and the module 1031 is 100 mm. In this case, a delay time fromtransmission of a DPS(W) to reception of a DPS(R) having a correspondingphase at the memory controller (MC) 1011 is 1400 ps, and a setup timeand a hold time for domain crossing in this case become 1400 ps and 1100ps, respectively, so that a sufficient timing margin can be obtained.

In FIG. 66, the DPS(W) is transmitted to the DRAMs 110 from the memorycontroller (MC) 1011, and the DPS(R) having the same phase as thereceived DPS(W) is transmitted from the DRAMs 110 to the memorycontroller (MC) 1011.

Therefore, it is understood that this embodiment employs the systemwherein the DPS is transmitted bidirectionally on the same DPS wiring.Thus, actually, the configuration is employed wherein the memorycontroller (MC) 1011 and each DRAM 110 transmit the DPS alternately, andthe internal clock signal is reproduced based on the received DPS.

Further, in the example shown in FIG. 64, two pairs of command/addresssignals and clock signals are produced from the memory controller (MC)1011 relative to the memory module 1031. On the other hand, likeoperations can be achieved when a pair of command/address signal andclock signal are produced from the memory controller (MC) 1011.

Referring to FIG. 67, a memory system 1000 according to a fifth exampleof the present invention is provided with a configuration in which nineDRAMs 110 (1) to (9) are mounted on a module 1031 like in FIG. 64,wherein a command/address signal and a clock signal that are common toall the DRAMs 110 are fed from a memory controller 1011 to those nineDRAMs 110 via a left end of the module 1031. That is, the nine DRAMs 110share the command/address signal and the clock signal. In this case,assuming that the propagation delay occurs like in FIG. 64, apropagation delay difference of (728+910) ps (=1638 ps) occurs in thecommand/address signal and the clock signal relative to a data signal DQat the farthest end DRAM 110 (9). Even if the domain crossing isimplemented with the period of 2500 ps of the clock signal subjected tothe frequency division to half it is difficult to ensure a timing marginfor domain crossing that is sufficient for dealing with such a largepropagation delay difference. For ensuring a timing margin forsufficient domain crossing, it is considered to use clocks having aperiod that is longer than that obtained by the frequency division tohalf.

On the other hand, as another technique to ensure a sufficient timemargin necessary for domain crossing while using the clocks subjected tothe frequency division to half, it is considered to divide the DRAMs 110on the module 1031 into two groups (herein called “first and second DQchannels”) as shown in FIG. 67. In this case, in the memory controller(MC) 1011, phases of DPS(W) given to the first and second DQ channelsare mutually shifted relative to the clock signal. That is, in the shownmemory controller (MC) 1011, phase offset values of the DPS(W) relativeto the clock signal are set to values suitable for the first and secondDQ channels.

In the shown example, the phase of the DPS(W) is advanced by 90 degreesrelative to the clock signal for the first DQ channel, while the DPS(W)is transmitted in phase with the clock signal for the second DQ channel.

Referring to FIG. 68, description will be given about a write operationin the DRAMs 110 (1) to (4) belonging to the first DQ channel. First,the memory controller (MC) 1011 divides to half in frequency a referenceclock signal of 800 MHz generated by a clock generator 102, thereby toproduce a clock signal of 400 MHz. This clock signal is fed to the DRAMs110 (1) to (4) belonging to the first DQ channel via clock wiring. Thememory controller (MC) 1011 further feeds a write command WRT ontocommand/address wiring synchronously with the produced clock signal.

On the other hand, DPS(W) are fed to the DRAMs 110 (1) to (4) of thefirst DQ channel via DPS wirings each having a length of about 100 mm.In this case, as clear from FIG. 68, the phase of the DPS(W) is advancedby 90 degrees (i.e. 625 ps) relative to the phase of the clock signal.

The DPS(W) produced at the memory controller (MC) 1011 arrive at theDRAMs 110 (1) to (4) of the first DQ channel via the DPS wirings. On theother hand, the clock signal and the write command (WRT) arrive at theDRAMs 110 (1) to (4) of the first DQ channel via the clock wiring andthe command/address wiring. Inasmuch as the clock wiring and thecommand/address wiring are each longer than the DPS wiring, apropagation delay time of the clock signal and the write command (WRT)becomes long, so that a propagation delay time difference between theDPS and the write command (WRT) is increased to 807 ps at the DRAM 110(1). At the DRAM 110 (1), a DRAM internal command is produced at a timeinstant where 1693 ps has elapsed after reception of the WRT. This meansthat, at the DRAM 110 (1), the write command (WRT) matched with theclock signal is caused to match with the timing of the received DPS.

Further, among the DRAMs 110 belonging to the first DQ channel, apropagation delay time difference between the DPS(W) and the clocksignal at the far-end DRAM 110 (4) becomes 1353 ps. Also in this case,by matching the write command (WRT) with the timing of the DPS, a timemargin of 1147 ps can be ensured. With this time margin, it is possibleto ensure a setup time and a hold time necessary for domain crossing.

Referring to FIG. 69, there is shown a read operation in the DRAMs 110(1) to (4) belonging to the first DQ channel. Also in this example, aread command (RED) is fed to the DRAMs 110 (1) to (4) from the memorycontroller (MC) 1011 synchronously with the clock signal, and a DPS isproduced as being advanced by 90 degrees relative to the clock signal,which is like the case of the write operation. Here, assuming that adistance between the memory controller (MC) 1011 and the module 1031 is100 mm, and a signal unit propagation time tPD is 7 ps/mm, the DPSarrives at the DRAM 110 (4) after 700 ps. The DRAM 110 (4) causes theread command (RED) to match with the UPS to thereby produce an internalread command, and transmits a DPS(R) to the memory controller (MC) 1011.This DPS(R) is received at the memory controller (MC) 1011 after a lapseof 1400 ps subsequently to the production of the DPS(W). A data signalDQ from the DRAM 110 (4) is received at the memory controller (MC) 1011at timing matched with the DPS(R).

By domain-crossing the timing of the received DPS(R) to the timing ofthe DPS(W), the memory controller (MC) 1011 causes the timing of thedata signal DQ to match with the timing of the DPS(W). By this, a timemargin of (1400+1100), i.e. 2500 ps, can be obtained also during theread operation.

Now, referring to FIG. 70, description will be given about a writeoperation of the DRAMs 110 (5) to (9) belonging to the second DQ channelin the memory system 1000 shown in FIG. 67. As clear from FIG. 70, withrespect to the second channel, the memory controller (MC) 1011 producesa clock signal of 400 Hz and a write command WRT matched with the clocksignal, and further produces a DPS(W) having the same phase as the clocksignal. In this manner, in this example, an offset value correspondingto 90 degrees of the clock signal is set between the DPS(W) for theDRAMs 110 (5) to (9) belonging to the second DQ channel and the DPS(W)for the DRAMs 110 (1) to (4) belonging to the first DQ channel, therebyto enable domain crossing even if there is a large propagation delaydifference between the clock signal and the data signal DQ.

Specifically, the clock signal (CLK) and the WRT from the memorycontroller (MC) 1011 arrive at the DRAMs 110 (5) to (9) of the second DQchannel via the long wirings, while the DPS(W) are given to the DRAMs110 (5) to (9) via the relatively short DPS wirings. In FIG. 70,operations of only the DRAMs 110 (5) and (9) are shown.

As clear from FIG. 67, the DPS(W) reaches the DRAM 110 (5) 910 psearlier than the clock signal and the WRT and, after 1590 ps, is causedto match with the DPS(W) received at the DRAM 110 (5). Therefore, at theDRAM 110 (5), it is possible to ensure a setup time and a hold timenecessary for domain crossing.

On the other hand, as clear from FIG. 67, the clock signal and the WRT,after having been produced at the memory controller (MC) 1011, arrive atthe DRAM 110 (9) with a delay of 1638 ps relative to the DPS(W). At thefarthest-end DRAM 110 (9), the received WRT is caused to match with thereceived DPS(W), thereby to produce an internal command. In this event,since there is a time margin of 862 ps between the WRT and DPS(W), it isseen that a setup time and a hold time necessary for domain crossing areensured.

Referring to FIG. 71, description will be given about a read operationin the DRAMs 110 (5) to (9) of the second DQ channel. Also in this case,a clock signal and a read command (RED) are transmitted, in phase withDPS(W), to the DRAMs 110 (5) to (9) from the memory controller (MC)1011.

Among the DRAMs 110 of the second DQ channel, the DPS(W) arrives at thefarthest-end DRAM 110 (9) 1638 ps earlier than the RED like in case ofthe WRT. As a result, the RED is caused to shift from the timing of theclock signal to the timing of the DPS(W) received at the DRAM 110 (9).

On the other hand, when the DPS(W) is produced at the memory controller(MC) 1011, the DPS(W) reaches the DRAM 110 (9) after a lapse of 700 ps,and the received DPS(W) is, as it is, transmitted to the memorycontroller (MC) 1011 from the DRAM 110 (9) as a DPS(R), so that a DPS(R)delayed by 1400 ps is produced at the memory controller (MC) 1011.

A data signal DQ from the DRAM 110 (9) is transmitted to the memorycontroller (MC) 1011 at timing of the DPS(R). At the memory controller(MC) 1011, as shown in FIG. 71, the data signal DQ transmitted at thetiming of the DPS(R) is caused to match with the timing of the DPS(W) inthe memory controller (MC) 1011. A time margin at this time is, as shownin the figure, 2500 ps, and therefore, it is seen that a time marginsufficient for performing domain crossing can be ensured.

As described above, although a time difference corresponding to theoffset is generated between the channels with respect to the read datasignals DQ in the memory controller (MC) 1011, a time margin necessaryfor domain crossing from the DPS(R) to the clock phase is sufficientlyensured.

As described above, since the memory controller 1011 operates inresponse to the system clocks from the clock generator 102 so as toachieve the operations like the buffer in the first to third examples,the global clocks and the system clocks given to the buffer and thememory controller 1011 can be collectively called main clocks.

In the present invention, a memory system includes a memory controllerand a module mounted with memory circuits and a buffer. Wiring includingdata wiring between the memory controller and the memory circuits on themodule is achieved via the buffer, and wiring including data wiringconnects buffers on modules in case mode. Accordingly, it is notnecessary to branch the wiring per module, and therefore, reflectionscaused by impedance mismatching can be prevented to enable a system thatcan operate at high speed at high frequencies. Further, according to thepresent invention, a transmission speed between the memory controllerand the buffer is set to be higher than a transmission speed between thebuffer and the memory circuits. This makes it possible to increase thenumber of modules to be connected to the memory controller. Further, itis possible to configure a system that does not rely on a write/readspeed of the memory circuits.

According to one embodiment of the present invention, not only the datawiring, but also the clock wiring and the command/address wiring connectbuffers on the modules from the memory controller. This can makesubstantially equal distances between the memory controller and therespective memory circuits mounted on the module. Therefore, timingdifferences caused by a different delay time per wiring can be avoided.Further, according to another embodiment of the present invention, byproviding a plurality of buffers on each module and connecting eachbuffer to memory circuits on the module, a load applied to each bufferand the wiring can be dispersed. Further, according to anotherembodiment of the present invention, memory circuits to be selectedsimultaneously are disposed over a plurality of modules, and a buffer ofeach module is individually connected to a memory controller. This makesit possible to disperse a load applied to each buffer, withoutincreasing the number of buffers.

1. A memory system comprising: a memory controller outputting a firstsignal and a second signal; a plurality of memory circuits receivingsaid first and second signals; a first wiring coupled between saidmemory controller and each of said plurality of memory circuits commonlyto transfer said first signal to said each of said plurality of memorycircuits in common; a plurality of second wirings provided correspondingwith said plurality of memory circuits, each of said plurality of secondwirings being coupled between said memory controller and an associatedone of said plurality of memory circuits independently to transfer saidsecond signal to an associated one of said plurality of memory circuits;a detection circuit which detects a skew between said first signal andsaid second signal; and an adjustment circuit which adjusts arelationship between said first signal and said second signal based onsaid skew.
 2. The memory system according to claim 1, wherein said firstsignal is a clock signal or a command/address signal, said second signalis a data signal or a timing signal being synchronized with said datasignal.
 3. The memory system according to claim 1, wherein said firstsignal is a clock signal, said second signal is a data signal.
 4. Thememory system according to claim 1, wherein said first signal is acommand/address signal, said second signal is a timing signal beingsynchronized with a data signal.
 5. The memory system according to claim1, further comprising an output circuit which outputs a third signalfrom each of said plurality of memory circuits to said memorycontroller, said third signal being based on said first signal or saidsecond signal.
 6. The memory system according to claim 5, wherein athird wiring for transmitting said third signal is connected directlybetween said memory controller and each of said plurality of memorycircuits.
 7. The memory system according to claim 5, wherein saidadjustment circuit adjusts said relationship based on said skew byutilizing said third signal.
 8. The memory system according to claim 1,wherein said plurality of memory circuits includes a first and a secondmemory circuits, a length of said first wiring connected between saidfirst memory circuit and said memory controller is different from alength of said first wiring connected between said second memory circuitand said memory controller, a length of said second wiring connectedbetween said first memory circuit and said memory controller issubstantially identical with a length of said second wiring connectedbetween said second memory circuit and said memory controller.
 9. Thememory system according to claim 8, wherein a size of said skew in saidfirst memory circuit is different from a size of said skew in saidsecond memory circuit.
 10. A memory module comprising: a substrate; aplurality of memory circuits mounted on said substrate, said pluralityof memory circuits receiving a first signal and a second signal; and adetection circuit which detects a skew between said first signal andsaid second signal.
 11. The memory module according to claim 10, furthercomprising a first wiring transmitting said first signal connectedcommonly to all of said plurality of memory circuits, a second wiringfor transmitting said second signal connected directly to each of saidplurality of memory circuits.
 12. The memory module according to claim10, wherein said first signal is a clock signal or a command/addresssignal, said second signal is a data signal or a timing signal beingsynchronized with said data signal.
 13. The memory module according toclaim 10, further comprising an output circuit supplying a third signalbased on said first signal or said second signal.
 14. A memorycontroller comprising: a first output circuit and a second outputcircuit outputting a first signal and a second signal, respectively; andan adjustment circuit which adjusts a relationship between said firstsignal and said second signal.
 15. The memory controller according toclaim 14, further comprising an input circuit receiving a third signal.16. The memory controller according to claim 15, wherein said adjustmentcircuit adjusts said relationship between said first signal and saidsecond signal by utilizing said third signal.